∫[0，a] √宽纳a^2 -x^2dx首先算不定积分∫√（a^2 -x^2）dx设x=asinu,dx=acosudu,√（a^2 -x^2）=acosu∫码搏√（a^2 -x^2）dx=∫acosuacosudu=∫a^2cos^2udu=∫慎模没a^2(1+cos2u)/2du =a^2/4∫(1+cos2u)d(2u)=a^2/4[2u+sin2u]
