√(81+x^2)
= √(81+81tan^2θ)
= √(81(1+tan^2θ))
= √(81sec^2θ)
= 9secθ
Looks like you need to brush up on your basic trig identities.
If x = 9tanθ, use trigonometric substitution to write square root 81 + x^2 as a trigonometric function of θ, where -pi/2 < θ < pi/2
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