已知函数f(x)=cosx-cos(x+π/2) x∈R.若f（x）＝3分之4.求sin2x的值

f(x)=cosx-cos(x+π/2)=cosx+sinx
f(x)*f(x)=cosx*cosx+2sinx cosx+sinx*sinx=1+sin2x=3/4
sin2x=-1/4