>>419
a ≠ 0のとき
x = at
とおくと
dx/dt = a
∫1/(x^2+a^2)dx = ∫1/((at)^2 + a^2) (dx/dt) dt
= (1/a) ∫1/(t^2 +1) dt
= (1/a) arctan(t) +c