∫√（a^2-x^2）dx设x=asint则dx=dasint=acostdta^2-x^2=a^2-a^2sint^2=a^2cost^2∫√（a^2-x^2）dx=∫acost*acostdt=a^2∫cost^2dt=a^2∫（cos2t+1）/2dt=a^2/4∫(cos2t+1)d2t=a^2/4*(sin2t+2t)将x=asint代回∫√（...