∫（π/4, 0）（tanθ）^2dθ
=∫（π/4, 0）[（secθ）^2-1] dθ
=∫（π/4, 0）（secθ）^2 dθ - ∫（π/4, 0）1 dθ
=tanθ|(π/4, 0)-(π/4, - 0)
=1 - π/4