S = πr√(r^2+h^2)
h = 8 m (constant)
An approximation of S when r changes from 9 to 8.9
Such an approximation is usually made by estimating the change using the first derivative. That derivative with respect to r is
... S' = π√(r^2+h^2) + πr(1/2·r)/√(r^2+h^2)
... S' = π(2r^2 +h^2)/√(r^2 +h^2) . . . . . use a common denominator
For r=9, h=8, this is
... S' = π(2·81 +64)/√(81+64) = 226π/√145 ≈ 58.96
Then the change in lateral surface area will be approximately
... ∆S ≈ (∆r)·S' ≈ (-0.1)·(58.96) ≈ -5.90 . . . m²
