Answer: No, the side lengths do not form a right triangle.
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Work Shown:
a^2+b^2 = c^2
7^2+20^2 = 21^2
49+400 = 441
449 = 441
We end up with a false equation, so the original equation is false for those a,b,c values.
By the pythagorean theorem converse, we do not have a right triangle.
Because a^2+b^2 > c^2, this triangle is acute.
