(sinxcosy)(f(2x+2y)−f(2x−2y))=(cosxsiny)(f(2x+2y)+f(2x−2y))f(2x+2y)(sin(x−y))=f(2x−2y)sin(x+y)sin(x+y)f(2x+2y)=sin(x−y)f(2x−2y) Put 2x+2y=m,2x−2y=nsin(2m)f(m)=sin(2n)f(n)=K⇒f(m)=Ksin(2m)∴f(x)=Ksin(2x)f′(x)=2Kcos(2x)PPut x=0;21=2K⇒K=1f′(x)=21cos2x
f′′(x)=−41sin2x4f′′(35π)=(−41sin(65π))24=8−24=−3