Suppose we have a triangle with a right angle at its height, with side a, 10 inches, side b, unknown, and side c, 24 inches; inscribed in a semi-circle. enter image description here

Now, I'm asked to find the area of the semi circle only, in terms of $\pi$. I was already given the answer, which is $84.5\pi -120 \ in^2$, but I don't understand how to get this conclusion.

I tried finding the area this way, but I get a pretty different answer: $A=\frac{\pi r^2}{2}, r=12 \ inches$


$$A=72\pi \ in^2$$

Then I know I have to subtract the triangle because I only want the area of the shaded region (the semi-circle), so



So I end up getting an answer of $A=72\pi-2\sqrt{119} \ in^2$ which is clearly false. Could anyone explain where I went wrong?


The question is poorly worded - it is not clear which side of the triangle is the longest (the hypotenuse).

You are assuming that the 24 inch side of the triangle is the longest side and is therefore the diameter of the semi circle.

The book answer is assuming that the unknown side of the triangle is the longest side. In this case the length of this side is $\sqrt{10^2+24^2}=26$ inches, the radius of the semicircle is $13$ inches, the area of the triangle is $\frac{10 \times 24}{2} = 120$ square inches and the area of the semicircle outside of the triangle is

$\frac{13^2\pi}{2}-120$ square inches.

  • $\begingroup$ Here is an image of the figure. ibb.co/2M2gwQG $\endgroup$ – Lex_i Dec 17 '18 at 13:03
  • $\begingroup$ @Lex_i Please include that in the question body rather posting it in a comment, and please use SE's built-in image upload because external links tend to be, in general, unstable. $\endgroup$ – GNUSupporter 8964民主女神 地下教會 Dec 17 '18 at 13:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.