3x=tany =>原式=(1/3)(secy)dy
(secy)dy=0.5(ln|1+siny|-ln|1-siny|)
siny=(3x)/根号(1+9x^2)
然后代入即可.注意后面加C
http://zhidao.baidu.com/link?url=h5hQIE_X7GX12udB4D2xOtTUyhORMFcpcKJx-mMRD3CVWBInwlIR7WKQrI3cHxrr8rtsCcH_MbFQY7BlXN7NHoqpEV9q5WQFR5o3POvOOUe