令x=asint
则定积分等于∫(0,π/2)asint*acostdasint （换元法）
= a^3∫ (0,π/2)sint*cos²tdt
= - a^3∫ (0,π/2)cos²tdcost （凑微分法）
= - (1/3)a^3(cost)^3|∫(0,π/2)
= (1/3)a^3