SOLUTION: If a triangle has the measures 4,7 and 9 is it an acute, obtuse or right?

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Question 382794: If a triangle has the measures 4,7 and 9 is it an acute, obtuse or right?
Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
Use the cosine law for the longest side of the triangle:
9%5E2+=+4%5E2+%2B+7%5E2+-+2%2A4%2A7cosA, where A is the angle opposite the side of length 9.
81 = 16 + 49 - 56cosA;
16 = -56cosA;
cosA = -16/56 = -2/7;
A = 106.6 degrees (approximately)
Therefore the triangle is an obtuse triangle.