785. -2cosx/2sinx/2*1/2=-cosx/2sinx/2=-sinx/2
y=sinx y'=cosx
-2cos3xsin3x*3+2sin3xcos3x*3=6(-cos3xsin3x+sin3xcos3x)=0
y=-1/2sinx y'=-1/2cosx
786 y=sin(2x-x)=sinx y'=cosx
y=sin(x/3+2x/3)=sinx y'=cosx
y=cos(3x-2x)=cosx y'=-sinx
y=cos(x/5+4x/5)=cosx y'=-sinx
