6 Different Angles

Geometry Level 2

What is the sum of the 6 blue angles?

18 0 180^\circ 36 0 360^\circ 45 0 450^\circ 54 0 540^\circ

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49 solutions

Chung Kevin
Nov 11, 2016

The blue angles are equal to the sum of the angles in the 3 pink triangles, minus the sum of the angles in the green triangles. Hence, the sum is

3 × 18 0 18 0 = 36 0 3 \times 180^ \circ - 180^ \circ = 360 ^ \circ

I am not convinced that the pink triangle is an equilateral triangle. since there are no parallel lines here. But since it is the SUM of its angles which is important, and since this will always equal 180 degrees, it doesn't matter whether it is or not. John Oakes

John Oakes - 4 years, 4 months ago

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Right, we do not need the assumption that the pink triangle is equilateral :)

Chung Kevin - 4 years, 4 months ago

I didn't understand anything of your solution.

Anik Saha - 4 years, 6 months ago

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Do you understand what the sum of angles of a pink triangle are? Do you see why it gives us 2 of the blue angles and 1 of the angles in the green triangle?

Chung Kevin - 4 years, 6 months ago

You took the words right out of my mouth! :)

Hannah Park - 4 years, 6 months ago

It doesn't matter if the inner triangle is equilateral. It could have been 40, 60, 80, or whatever It would still be one of each of the triangle's angles for them to each add up to 180 and so the inner one does as well.

Kristian Valle - 2 weeks, 3 days ago

The sum of all the angles within a triangle (any type) is 180°. The triangle created by the intersection of the three triangles is an equilateral triangle (each angle is the same), so 60°. So 180°+180°+180°-60°-60°-60°= 360°

Makes sense?

Sam Fis - 4 years, 4 months ago

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It need not be true that the small pink triangle is equilateral. The result is still true, as John mentioned.

Chung Kevin - 4 years, 4 months ago
Ojasee Duble
Dec 21, 2016

In all there are 3 big triangles & 1 small triangle at the centre.

Sum of measure of angles of all 3 triangles = 180º + 180º + 180º = 540º

But the small triangle at the centre too measures - 180º

(This small triangles has 3 acute angles of the larger triangles)

Thus, Sum of 6 blue angles = 540º - 180º = 360º

Ans. 360º

Nice! That feels so quick :)

Chung Kevin - 4 years, 5 months ago

Great solution!

Kerushan Govender - 4 years, 4 months ago
Angel Krastev
Jun 30, 2019


3x180 -180=360 Here is how:

BLUE+RED=540
RED=180

BLUE=540-180=360

Arjen Vreugdenhil
Nov 21, 2016

When traversing the diagram, you turn a total of two revolutions, or 72 0 720^\circ , before returning at the starting point. This is the sum of the external angles. Since internal angle = 18 0 external angle , \text{internal angle} = 180^\circ - \text{external angle}, and there are six internal angles here, we have internal angle = 18 0 external angle = 6 × 18 0 72 0 = 36 0 . \sum\text{internal angle} = \sum 180^\circ - \sum\text{external angle} = 6\times 180^\circ - 720^\circ = \boxed{360^\circ}.

I like this too! So many nice ways to view this problem :)

Chung Kevin - 4 years, 6 months ago

?????? not understood

Anik Saha - 4 years, 6 months ago

Turtle Geometry by Abelson and diSessa is a wonderful book - where taught now I wonder?

Chris Wallace - 4 years, 4 months ago

With a physical straightedge or mental one you can just traverse/pivot on the outer angles and get 360 without even thinking about the inner angles. In all honesty I think it's more fun/easier to just pivot on the relevant angles.

Brian Bohan - 1 year, 11 months ago

I think you could also consider the angle a vector moving around the path would rotate through to get back to the start.

Tom Mason - 1 year, 8 months ago

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That is what I did.

Arjen Vreugdenhil - 1 year, 8 months ago
Peter Macgregor
Nov 21, 2016

I held my pen above the line at the right hand side of the diagram. I then rotated it through the acute angles at the top right, bottom left, bottom right, top left, middle left and middle right of the diagram so that after each rotation the pen lined up with another line of the diagram. After passing through all the angles, the pen came back to its original position, having made one complete rotation of 36 0 \boxed{360^\circ} .

You can gain some confidence in this method by using it to show that the sum of the angles in a triangle is 18 0 180^\circ and in a quadrilateral is 36 0 360^\circ .

I like the "rotate a pen" idea!

Chung Kevin - 4 years, 6 months ago
Terry Jones
Feb 22, 2017

3X360=540 (3 large triangles) 540 - 180 (common small triangle) = 360

Wade Gruber
Feb 19, 2017

Outer angles = (180 - x) + (180 - y) + (180 - z)

x + y + z = 180

Therefore,

(3 X 180) - 180 = Answer

540 - 180 = 360

Nathaniel Vivero
Jan 26, 2017

360, perfect triangle in the middle which means, 180/3=60. so 180 degrees per triangle minus 60 = 120. which means per triangle has a remaining angle of 120. 120*3= 360 degrees.

Well done :)

Chung Kevin - 4 years, 4 months ago

Just because it 'looks like' an equilateral triangle does not mean that it is one absolutely. Therefore, none of the angles in the diagram is absolutely determined. Regardless, the sum of the angles of the central triangle is still 180 degrees.

Patrick Conoley - 4 years, 4 months ago
Revan Caluya
Jan 25, 2017

3(big triangles) x 180 = 540

1(small middle triangle) = 180

Deducting the sum of angles from the middle triangle will give you the sum of the 6 angles.

540 - 180 = 360

Well done :)

Chung Kevin - 4 years, 4 months ago

That's how I did it too (but without the first calc) - err, (3-1) x 180 = 360.

Brandon Ashworth - 4 years, 4 months ago
Alistair Swodeck
Feb 23, 2017

There are three triangle. The sum of the angles inside a triangle is 180. 3 x 180 = 540. The sum of the non blue angles is 180. Therefore 540 - 180 = 360

Mike Miller
Feb 22, 2017

the sum of all 3 triangles angles is 540, and you know that 3 of thoes angles are not accounted for, so it must be 540-180=360

Mazyar Zagros
Feb 11, 2017

totals 360

Although the diagram 'looks' like a solution, do we really know that the sides will make a quadrilateral after flipping?

Patrick Conoley - 4 years, 4 months ago
Maria Lobato
Jan 8, 2017

180=180-ang1'-ang2'+180-ang1''-ang2''+180-ang1'''-ang2''' ----- 3*180-180= sum angles

Oh, that's a nice equation to put together!

Chung Kevin - 4 years, 5 months ago
Magdy Abo-bakr
Jan 8, 2017

the sum of all angles in three triangles =3*180, and the 6 angles represent that sum -the sum of angles in the small triangle which is equal 360

Nice! That's what I did too :)

Chung Kevin - 4 years, 5 months ago
Edward Rogers
Jan 8, 2017

Angle 1 = Angle 2 + Angle 3 (Exterior Angles of a triangle equal the sum of the 2 remote interior angles)

Angle 4 = Angle 5 + Angle 6 (Exterior Angles of a triangle equal the sum of the 2 remote interior angles)

Angle 1 + Angle 4 + Angle 7 + Angle 8 = 360° (The sum of the interior angles of any 4 sided polygon equals 360°)

Angle 2 + Angle 3 + Angle 5 + Angle 6 + Angle 7 + Angle 8 = 360° (Substitution)

Nice way of presenting it!

Chung Kevin - 4 years, 5 months ago
Toby Knipping
May 13, 2020

Imagine walking along the lines. You walk, turn, walk, turn again in the same direction, walk, turn again in the same direction etc. If you end up back where you started then one way or another you’ve gone in a circle. (360deg).

central triangle is an equilateral, thus each angle is 60°. Each central triangle's angle is also an angle of the other 3 triangles. Thus an easy subtraction gives you the answer.

Steve Shaff
Jun 30, 2019

Apply the Exterior Angle Theorem for a triangle twice, then use the fact that the sum of the measures of the angles of a quadrilateral is 360. Easy!

Oin Vitew
Feb 24, 2017

Sum of the three large triangles (3*180) minus sum of the small inner triangle formed (180)

Peter Gorman
Feb 24, 2017

I just know man.

Donald Deal
Feb 24, 2017

Because the angles encompass the exterior angles- it is effectively the pie pieces cut out of a hypothetical circle. They have to be 360 degrees, a complete boxing of the compass.

Peter Taylor
Feb 24, 2017

The shape is made of 3 triangles - seperate these triangles and the sum of all the angles is 3 x 180 = 540 deg when we overlap the triangles again we get a smaller triangle created - this is made up of 3 of the previous angles. the small triangle angles add up to 180 degrees - so we subtract this from the total so 540 -180 = 360 QED

3 180 = sum of angles in triangles with blue angles, that sum includes sum of angles of their intersections which is 180, therefore (3-1) 180 = 360

Casey W
Feb 23, 2017

Visualize 3 triangles intersecting (two of the blue angles and a third unshaded). Starting with triangle in top left and moving clockwise you have triangle abC, deF, & ghI (lower case are shaded blue, upper case is not). From basic geometry we know the 3 angles of each triangle add to 180.

Start with the triangle in the top left. Label its angles a, b, & C. Angle C is created from two intersecting lines. Either of the adjacent angles is equal to 180 - [C]. Naturally this adjacent angle must be equal to the sum of two shaded angles of the triangle, (that is, a+b).

Notice now that you have a quadrilateral in bottom left: it has angles g & h, as well as the angle a+b that you derived, and finally a fourth angle yet undetermined. This fourth angle can be determined similarly to the 180 - [C] = (a+b) angle we derived and it is found to be d+e.

So we are left with a quadrilateral that has the four angles: g, h, (a+b), & (d+e). Like a triangle's angles add up to 180, a quadrilaterals add to 360. Therefor g + h + (a+b) + (d+e) must equal 360.

Hope LeGrande
Feb 23, 2017

Each angle of the three outer triangles are equal to one angle of a triangle next to it. And to one angle of the small interior triangle. So basically it's 180 × 2, because each angle degree appears twice.

John Woolley
Feb 21, 2017

Each of the three large triangles has angles adding to 180, of course, so the six blue angles plus the three angles of the center triangle add up to 3x180. But the center triangles' angles add up to 180, so the sum of the blue angles is 2x180, or 360.

Neketh Aithal
Feb 20, 2017

Each triangle sums to 180 degrees. So, for each triangle, simply represent the sum of the blue angles with a variable and solve for the third angle: 1) A1 = 180 - A. 2) A2 = 180 - B. 3) A3 = 180 - C.

Keep in mind that the Sum A+B+C represents the sum of all blue angles.

Also notice that these three angles are part of the smaller, center triangle, so their sum total has to be 180 degrees:

A1+A2+A3 = 180,

Giving us:

540 - (A+B+C) = 180,

Which after simplification gives:

A+B+C = 360

Thus the total value represented by the three angles is 360 degrees.

John Baxendale
Feb 19, 2017

There are three overlapping Triangles 1, 2, and 3 . Consider 1 with marked angles 1 and 2 with Apex angle =x, therefor x=180 - (1+2 ) Hence (1+2) = 180-x . Similarly (3+4)=180-y and (5+6)=180-z Add these three to give (1+2) +(3+4) +(5+6)= 3x180-(x+y+z), But (x+y+z) = 180 therefor (1+2)+(3+4)+(5+6)=(3x180)-180=2x180 =360

Xi Illenium
Feb 18, 2017

Twist the shape to a rectangle. And we all know rectangle is 360

Robert Briggs
Feb 16, 2017

regardless of starting point in order to return to starting point one must go a full circle so to speak

Evan Stenstrom
Feb 16, 2017

sum of 1 triangle is always 180 degrees, multiplied by 2 is 360

Adrian Lorent
Feb 15, 2017

If we call the angle of the internal angles of the small triangle a,b and c, and consider the fact that the internal angles of a triangle are 180 degrees, we then have 180-a+180-b+180-c=180x3 - (a+b+c). a+b+c=180, so the sum of the 6 blue angles is 180x3-180=360

Doug Jenney
Feb 13, 2017

Let a1,b1,c1 be the angles of triangle1, a2,b2,c2 for triangle 2, and a3,b3,c3 of triangle3, where c1,c2,c3 are angles of the smaller intersecting triangle 4.

  • Step 1a) c1+c2+c3=180.
  • Step 1b) c1=(180-(a1+b1)) c2=(180-(a2+b2)) c3=(180-(a3+b3))
  • Step 2) (180-(a1+b1))+(180-(a2+b2))+(180-(a3+b3))=c1+c2+c3=180
  • Step 3) 180-a1-b1+180-a2-b2+180-a3-b3=180
  • Step 4) 180+180+180-180=a1+b1+a2+b2+a3+b3
  • Step 5) 360 = a1+b1+a2+b2+a3+b3

Small triangle sum of angles: a + b + c = 180 a + b + c = 180

Upper left triangle: 180 ( α + β ) = a 180 - (\alpha + \beta) = a

Upper right triangle: 180 ( γ + δ ) = b 180 - (\gamma + \delta) = b

Lower triangle: 180 ( ϵ + ζ ) = c 180 - (\epsilon + \zeta) = c

Now, ( ( 180 ( α + β ) ) + ( ( 180 ( γ + δ ) ) + ( ( 180 ( ϵ + ζ ) ) = a + b + c = 180 \left( (180 - (\alpha + \beta) \right) + \left( (180 - (\gamma + \delta) \right) + \left( (180 - (\epsilon + \zeta) \right) = a + b+c=180

Solving for ( α + β ) + ( γ + δ ) + ( ϵ + ζ ) (\alpha + \beta) + (\gamma + \delta) + (\epsilon + \zeta) yields 360.

The sum of each triangle's angle is 180 degrees. The small triangle formed by the intersection of the three triangles is an equilateral triangle where all angles are 60 degrees. (180-60)3 = (120)3 = 360

The small triangle need not be equilateral. We're still subtracting off all of the angles, so we have 3 × 180 180 = 360 3 \times 180 - 180 = 360 .

Chung Kevin - 4 years, 4 months ago
William Grohs
Feb 8, 2017

The sum of the angles of a triangle is 180 degrees The triangle with no blue angles is made of the non blue angle of each of the other three triangles If their sum (180) is subtracted from the sum of the other tree (540) the result is 360

Tomaso Sarducci
Feb 6, 2017

You have two right triangles on the upper left and upper right.The lower angles have to be 90 degrees. The bottom triangle is an equalateral triangle so the two marked angles have to be 45 degree. So far that is 90 X 2 +45 X 2 which is 270 degrees. A simple observation of the legs on the upper right triangle show that the short one is half as long as the long one so the marked angle is 30 degrees.We are now up to 300 degrees worth of marked angles. The left upper triangle's remaining marked angle is twice as big as the one on the right therefore it has to be 60 degrees and the final answer is 360 degrees. Besides if you take all the blue markings out and place them together flat edge to flat edge they make a perfect circle and guess what, voila 360 degrees.

William Sandey
Feb 6, 2017

Let's assign labels to each of the angles:

The sum of A, B, and C is 180 degrees. Same goes for angles D, E, and F, as well as angles G, H, and I.

      A + B + C = 180
      D + E + F = 180
      G + H + I = 180

So now we can calculate the sum of all the angles:

      (A + B + C) + (D + E + F) + (G + H + I) = X

But we only want the blue shaded ones. So we have to remove the three angles that make up the smaller, center triangle:

      (A + B + C) + (D + E + F) + (G + H + I) - (A + D + G) = X

We don't know the actual values of A, B, C, or any of the angles, but we do know what they add up to for each triangle - 180 degrees. Each triangle is represented by the grouping right able, so substituting 180 for each group, we get:

      180 + 180 + 180 - 180 = X

And then it's simple addition and subtraction from there:

      180 + 180 + 180 - 180 = 360
Jacob Butler
Feb 4, 2017

Taking any one side, it is effectively turning through 360, albeit displaced and extended.

Rachel Shaw
Feb 4, 2017

i saw it as 2 triangles in my head and figured 180 degrees in each triangle times 2

Matt Lewton
Feb 2, 2017

There are three large triangles each at 180 degrees, one angle of each provides an angle to the smaller internal triangle also required to add up to 180 degrees. Therefore 3 x 180 = 540. 540 - 180 = 360. They remaining 6 angle must add up to 360 total degrees.

Bill Henry
Feb 2, 2017

3*180 -(x+y+z) , x+y+z = 180, 2(180) = 360

James Palmer
Feb 1, 2017

middle is an equilateral triangle, each internal angle of middle triangle completes 3rd corner of a triangle with two blue angles, therefore sum of each pair of blue angles = 120

What makes you believe that the middle triangle is an equilateral triangle? It isn't forced to be.

Charles Gaskell - 4 years, 4 months ago

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It actually doesn't matter what type of triangle it is, since it still has the same sum of internal angles, each of which completes one of the larger triangles. So it will always be (180*3) - 180 = 360 regardless of how the internal angles of the middle triangle are distributed.

James Palmer - 3 years, 1 month ago

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Agree with that. The bit about the sum of each pair of blue angles being 120 degrees is (I think) incorrect

Charles Gaskell - 2 years, 11 months ago
Jeremy Stanger
Feb 1, 2017

Each angle has a complementary one such that if we move a corner along one of the three 'long lines', its complementary angle changes such that the sum of the two is constant. Thus, moving a corner in such a way, we keep the sum of the angles constant. It is trivial to move points around like this such that the lines form two triangles on top of each other at the centre (the small triangle we see already). Thus, the angles sum to twice the sum of the angles in a triangle: 360 degrees.

Of course, we never appealed to the specific case shown, so in fact one could move any points around however one likes in order to construct a 'double triangle' and the result would still hold.

Nice interpretation :)

Chung Kevin - 4 years, 4 months ago
Byron Johnson
Jan 31, 2017

All of the triangles can be seen as one contigious figure if they are seen to be connected by one continous line, and the sum of the angles in any contigious figure is always 360 degrees.

Steven Aasen
Jan 31, 2017

The sum of the internal angles of a triangle is 180 ((3-2) 180). The smallest triangle contains the three angles missing from each of the "larger" triangles. Therefore the sum is 3 180 (for the three larger triangles) -180 (the sum of the missing angles formed by the smallest triangle. Or 540-180=360 degress

Mark Leone
Jan 27, 2017

From the diagram, we see that

1 + 2 + C = 180

3 + 4 + A = 180

5 + 6 + B = 180

A + B + C = 180

Now from the above equations, we see that

1 + 2 + 3 + 4 + 5 + 6 = 180 + 180 + 180 - (C + A + B) = 180 * 3 - 180 = 360

Another way:

Rotate line XY clockwise through all the angles in the following sequence: 2 5 6 3 4 1. The line made one complete revolution; so the sum of those angles is 360.

Chris Burchfiel
Jan 26, 2017

The sum of the angles in a triangle is always 180, so the measure of all the angles of all the triangles would be 180x3=540. BUT! We aren't counting the angles that fall inside the little central triangle, which (since they themselves form a triangle) add up to 180. Thus, the answer is 180 x 2 = 360!

Sky Moon
Jan 26, 2017

there are three big triangles and one small then small triangle is an Equilateral triangle so all the big triangles having miss an angle of 60...... so 120+120+120=360

We are asked to be creative. Unfold the drawing and you see a rectangle. In doing so, six angles "become" four angles....four corners angles of a rectangle: 90 + 90 + 90 +90 = 360. Just sayin'.

Jeff Kadas - 4 years, 4 months ago

I "unfolded" the diagram and made a rectangle. a rectangle has a total of 360 degrees

Tobias Nash - 4 years, 4 months ago

Is the middle triangle an equilateral triangle? How do you know?

Charles Gaskell - 4 years, 4 months ago

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