例13求 cos xar 解Jc3xdr=+ cos 2 2 dx+ cos 2xdx dx+= cos 2xd2x x+sin 2x +C.cos . 2  求 xdx  xdx 2 cos  + = x x d 2 1 cos2 ( )   = dx + cos 2xdx 2 1       = +   x cos 2x 2x 2 1 2 1 d d sin2 . 2 1 2 1 x x + C      = + 例13 解