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x=a*sint,t=arcsin(x/a),dx=a*cost*dt不定积分x^2dx/((a^2-x^2)^(1/2))=Sa^2*(sint)^2 *a*cost *dt /a*cost=Sa^2 *(sint)^2 *dt=a^2 /2 *S(1-cos2t)*dt=a^2 /2 *t-a^2 /4 *sin2t+c=a^2 /2 *arcsin(x/a)-1/2*x*根号（a^2-x^2)+c
