SOLUTION: Is a triangle a right, acute, or an obtuse triangle if has the measures 11, 11, and 15?

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Question 256162: Is a triangle a right, acute, or an obtuse triangle if has the measures 11, 11, and 15?
Answer by Edwin McCravy(20056) About Me  (Show Source):
You can put this solution on YOUR website!
Is a triangle a right, acute, or an obtuse triangle if
has the measures 11, 11, and 15?
The only angle that could be obtuse or right in any triangle
is the angle opposite the longest side.

Let a = 15, b= 11, c = 11, then A is the angle opposite the 
longest side 15 

cos%28A%29+=+%28b%5E2%2Bc%5E2-a%5E2%29%2F%282bc%29
cos%28A%29+=+%2811%5E2%2B11%5E2-15%5E2%29%2F%282%2811%29%2811%29%29
cos%28A%29+=+.0702479339
A=%2285.97177216%B0%22

That's less than 90°, so it's acute!  Thus the triangle
is an acute triangle.

Edwin