Round this value however you need to. The exact length is sqrt(319)
In other words, sqrt(319) = 17.8606
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Work Shown:
Apply the pythagorean theorem to solve for b.
a^2+b^2 = c^2
9^2 + b^2 = 20^2
81 + b^2 = 400
b^2 = 400-81
b^2 = 319
b = sqrt(319)
b = 17.8606 which is approximate
I'm going with the approximate value it makes more sense to do so in this case. Saying "the length is sqrt(319)" to someone not familiar with math, would probably not convey just how long the third side must be. Round the value in bold however you need to.