dy=2sinxcosxdx=sin(2x)dx
y=0.5sin2x, dy=0.5cos2x*2dx=cos(2x)dx
y'=cos(3/x)(3/x)'=-(1/x²)cos(3/x)
y'=-sin(2/x)(2/x)'=(2/x²)sin(2/x)
