f(x)f(4) =cos(logx)*cos(log4)
=cos(logx)*cos(log4)*2/2
= { cos(logx+log4)+cos(logx-log4) }/2
{since 2cos(a)cos(b)=cos(a+b)+cos(a-b)}
={cos(log4x)+cos(log(x/4))}/2
{since logb(xy) = logbx + logby. and logb(x/y) = logbx - logby.}
={f(4x)+f(x/4)}/2---------------------------------------------------------------(1)
therefore
f(x)f(4) - 1/2{f(x/4) + f(4x)}=0 [from(1)]