The Ten Points

Geometry Level 3

Ten points Q , R , S , T , U , V , W , X , Y Q,R,S,T,U,V,W,X,Y and Z Z are equally and consecutively spaced on a circle. What is the size, in degrees, of the angle Q T W ? \angle QTW?

36 54 60 72

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2 solutions

Callie Ferguson
Oct 28, 2020

If the circle has ten points, that means that each angle will be 36 degrees because:

θ = \large{\theta=} 36 0 # interior angles = 36 0 10 = 3 6 \large{\dfrac{360 ^\circ}{\text{\# interior angles}} = \dfrac{360^\circ}{10} = 36 ^\circ}

The following formula is true for inscribed angles of this type:

m Q T W = 1 2 m Q C W \boxed{\text{m} \angle QTW = \dfrac{1}{2} \text{ m} \angle QCW}

We don't know Q T W \angle QTW , but we do know that each angle slice within the circle is 3 6 36 ^\circ . Since there are 4 of these slices within Q C W \angle QCW , we can see that:

Q C W = 3 6 ( 4 ) = 14 4 \angle QCW = 36 ^\circ (4) = 144 ^\circ

Now we can plug 14 4 144 ^\circ into the formula above to solve for m Q T W \text{m} \angle QTW :

m Q T W = 1 2 14 4 = 7 2 \large{\text{m} \angle QTW = \dfrac{1}{2} \cdot 144 ^\circ = 72 ^\circ}

So, the solution is 7 2 \boxed{\large{72^\circ}}

Bithiah Koshy
Oct 28, 2020

Simpler: since the points are equally spaced, and there are ten of them, each arc between consecutive points measures 36 degrees. There are four such arcs between Q Q and W W , so the arc Q W {QW} measures 144 degrees. The angle Q T W \angle QTW is inscribed in a circle, and thus it's measure is half that of the arc Q W {QW} it cuts. (Sadly, there is no "\arc" command in standard LaTeX.)

Richard Desper - 7 months, 2 weeks ago

@Bithiah Koshy , I've seen this question before, you copied it from AMC, there you go the website: website

Half pass3 - 7 months, 1 week ago

Lmao yea hehehehe

Bithiah Koshy - 7 months, 1 week ago

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