令x=6sint
原式=∫(0→π/2)cost*6costdt
=3∫(0→π/2)2cos^2(t)dt
=3∫(0→π/2)(cos(2t)+1)dt
=3/2sin(2t)|(0→π/2)+3t|(0→π/2)
=3/2π原式=∫(0→π/2)cost*6costdt不懂。。为什么不是=∫(0→π/2)cost*cost因为dx=d(6sint)=6costdt前面的根号开出来就是cost