Fit just nicely

Geometry Level 1

A B C ABC is an isosceles right triangle with A B = A C = 6 AB = AC = 6 .

Given that S D P F SDPF is a square, find the area of the square.


The answer is 8.

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10 solutions

Base from my figure above, we can use the derived formula

x = b h b + h \boxed{x=\dfrac{bh}{b+h}} note: I did not include the derivation.

b = 6 2 + 6 2 = 72 = 6 2 b=\sqrt{6^2+6^2}=\sqrt{72}=6\sqrt{2}

h = 6 2 ( 6 2 2 ) 2 = 18 = 3 2 h=\sqrt{6^2-\left(\dfrac{6\sqrt{2}}{2}\right)^2}=\sqrt{18}=3\sqrt{2}

Substitute the values on the formula.

x = ( 6 2 ) ( 3 2 ) 6 2 + 3 2 = 18 ( 2 ) 9 2 = 36 9 2 = 4 2 x=\dfrac{(6\sqrt{2})(3\sqrt{2})}{6\sqrt{2}+3\sqrt{2}}=\dfrac{18(2)}{9\sqrt{2}}=\dfrac{36}{9\sqrt{2}}=4\sqrt{2}

Solving for the area, we have

x 2 = ( 4 2 ) 2 = x^2=\left(\dfrac{4}{\sqrt{2}}\right)^2= 8 \boxed{8}

x=2*(2)^0.5 not 4/(2)^0.5

Vedanta Mohapatra - 3 years, 11 months ago

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can you post your solution? thanks ..

A Former Brilliant Member - 3 years, 11 months ago

side length of square = x. x sqrt{2}/2 + x sqrt{2} = 6 x sqrt{2} = 6/1.5 = 4 x = 4/sqrt{2} = 2 sqrt{2} x^2 = (2*sqrt{2})^2 = 8.

Jiya Dani - 1 month, 1 week ago
Seyed Seyedy
Apr 30, 2018

Just Genius......👌👌👌

Alif Ibn Akhtar - 7 months, 4 weeks ago
Akash Deep
Aug 11, 2014

its very easy , let SPDF have a side - x , in triangle FPC base angles each 45 because the big triangle is isosceles right angled triangle. so ,PC = FP = x similarly SD = BD = X and already PD = x BC = BD + PD + PC = 3X and by pythagorous theorem BC = 6 2 6\sqrt { 2 } , x = 2 2 2\sqrt { 2 } area = x 2 { x }^{ 2 } = 8

Nice solution! I did the same thing too. :)

I find the illustration misleading. I think the author should change "ABC=90 degrees" to m B A C = 9 0 m\angle BAC =90^{\circ} so that it matches the figure. :)

Jaydee Lucero - 6 years, 9 months ago

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thanks for improving my mistake

Sudhakar Yadav - 6 years, 8 months ago

i did it in the same way....nice solution

Nowroze Farhan - 6 years, 9 months ago
Mas Mus
Jan 9, 2017

Since A B C ABC is an isosceles right triangle, B = C = 4 5 \angle{B}=\angle{C}=45^{\circ} . Of course, B S F , F A P , and P D C \triangle{BSF},~\triangle{FAP}, \text{and}~\triangle{PDC} are isosceles right triangles too. Let A F = A P = x AF=AP=x , by pythagorous theorem we have F P = 2 x 2 FP=\sqrt{2x^2} and since S D P F SDPF is a square then S F = S D = D P = 2 x 2 SF=SD=DP=\sqrt{2x^2} . Now, look at the P D C \triangle{PDC}

( 6 x ) 2 = 2 x 2 + 2 x 2 6 x = 2 x x = 2 \begin{aligned}\left(6-x\right)^2&=2x^2+2x^2\\6-x&=2x\\x&=2\end{aligned}

Thus, the area of the square S D P F = 2 x 2 = 8 SDPF=2x^2=8 . _\square

Let x = ( S A ) x = (SA) . Then ( S D ) = ( S F ) = 2 x (SD) = (SF) = \sqrt{2} * x , and so ( S B ) = 2 x (SB) = 2x .

Thus ( A B ) = ( S A ) + ( S B ) = x + 2 x = 3 x = 6 x = 2 (AB) = (SA) + (SB) = x + 2x = 3x = 6 \Longrightarrow x = 2 .

The square S P D F SPDF thus has sides lengths 2 x = 2 2 \sqrt{2} * x = 2\sqrt{2} cm., and hence has an area of 8 c m 2 \boxed{8} cm^{2} .

How SB is 2x ??

gaurav bahl - 6 years, 2 months ago

Be careful the dimensions in the drawing are unitless.

A Former Brilliant Member - 4 years, 1 month ago
Akhila F.
Mar 10, 2018

Let the side of the square be x x

Since A B C ABC is an isosceles right angled triangle, A B C = A C B = 4 5 \angle ABC = \angle ACB = 45 ^ \circ

B F S = D P C = 18 0 ( 9 0 + 4 5 ) = 4 5 \angle BFS = \angle DPC = 180^\circ-(90^\circ+45^\circ) = 45^\circ

Therefore, both B S F BSF and P D C PDC are isosceles triangles and B S = D C = x BS = DC = x .

Hence, B C = B S + S D + D C = 3 x BC= BS+SD+DC = 3x

Using Pythagoras theorem in triangle A B C ABC ,

A B 2 + A C 2 = B C 2 AB^2 + AC^2 = BC^2

6 2 + 6 2 = ( 3 x ) 2 6^2+6^2 = (3x)^2

72 = 9 x 2 72 = 9x^2

x 2 = x^2 = Area of the square S D P F = 8 SDPF = \boxed{8} square units.

Triangle BSF and Triangle PDC are both 45- 45-90 triangle, meaning it's isosceles. BS=SF & PD = DC Let them be x, so the area of the square is x^2 BS+SD+DC=3x sqrt(2*6^2)=3x x= 2 sqrt (2) x^2=8 8 is the answer.

Yun Woonjoo
Dec 26, 2019

as you see this shape is made of 9 identical triangles. and the square is made of 4 of the triangles. the area of the triangle is 6*6/2=18. since the square takes up 4/9 of the triangle, 4/9 * 18 = 8

Josh Taylor
Dec 22, 2017

Triangle ABC has area 18 and can be split into nine congruent triangles (all of which are congruent to triangle AFP) each with area 2. Square FSPD is comprised of four of these triangles and therefore has area 8.

Based from my figure, let h h be the height and b b be the base of the triangle

Solving for the base, we have

b = 6 2 + 6 2 = 6 2 b=\sqrt{6^2+6^2}=6\sqrt{2}

Solving for the height, we have

h = 6 2 ( 6 2 2 ) 2 = 3 2 h=\sqrt{6^2-(\frac{6}{2}\sqrt{2})^2}=3\sqrt{2}

Now use the formula x = b h b + h x=\dfrac{bh}{b+h} (this is in reference with the figure on the left), we have

x = b h b + h = ( 6 2 ) ( 3 2 ) 6 2 + 3 2 = 4 2 x=\dfrac{bh}{b+h}=\dfrac{(6\sqrt{2})(3\sqrt{2})}{6\sqrt{2}+3\sqrt{2}}=\dfrac{4}{\sqrt{2}}

Finally, the area of the square is

x 2 = ( 4 2 ) 2 = x^2=(\dfrac{4}{\sqrt{2}})^2= 8 \large\color{#D61F06}\boxed{8}

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