y=3cos（π/2-x/4）cosx/4
=3*sin(x/4)*cos(x/4)
=3/2*sin(x/2),
而,-1≤sin(x/2)≤1.则有
y最大值=3/2,
Y最小值=-3/2.
函数y=3cos（π/2-x/4）cosx/4的值域[-3/2,3/2].