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Geometry Level 3

A circle with radius 217 is inscribed in a right triangle with leg lengths a a and b . b. The circumcircle of this triangle has radius 2017. What is a + b ? a+b?


The answer is 4468.

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2 solutions

Zach Abueg
Jul 2, 2017

With the circumcircle's radius r c r_c , we can see that the hypotenuse c c of the triangle is 2 × r c = 4034 2 \times r_c = 4034 .

Now, the radius r i r_i of the incircle of a right triangle with sides a , b a, b and hypotenuse c c is given by r i = a + b c 2 \displaystyle r_i = \frac{a + b - c}{2} :

r i = a + b c 2 a + b = 2 r i + c = 2 217 + 4034 = 4468 \displaystyle \begin{aligned} r_i & = \frac{a + b - c}{2} \\ \implies a + b & = 2r_i + c \\ & = 2 \cdot 217 + 4034 \\ & = \boxed{4468} \end{aligned}

Why is r_i given by this formula?

Muhammad Arafat - 3 years, 11 months ago

Using Thales' Theorem we get that 2017 = x + y 2 2017=\dfrac{x+y}{2} . Notice that r = 217 r=217 . From that

a + b = x + y + 2 r = 2017 2 + 217 2 = 4468 a+b=x+y+2r=2017*2+217*2=\boxed{4468}

Why is the hypotenuse x+y?

Muhammad Arafat - 3 years, 11 months ago

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Lol just figured it out it's because of the tangent lines

Muhammad Arafat - 3 years, 11 months ago

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