令x=asint dx=acostdt t=arcsin(x/a)原式=∫a^2cos^2tdt=a^2/2*∫(1+cos2t)dt=a^2/2*(t+sintcost)+C=a^2/2*arcsin(x/a)+ax/2*(1-x^2/a^2)^(1/2)+C
