f(x)=2acos²x+bsinxcosx-1
f(0)=2acos²(0)+bsin0cos0-1=1
2a-1=1 a=1
f(π/3)=2cos²(π/3)+bsin(π/3)cos(π/3)-1=-1/2+√3/2
1/2+b(√3/2)(1/2)=1/2+√3/2
b=2
f(x)=2cos²(x)+2sinxcosx-1
=cos(2x)+sin(2x)
=sin(π/2-2x)+sin(2x)
=2sin(π/4)cos(π/4-2x)
=√2cos(π/4-2x)>1
cos(π/4-2x)>√2/2
-π/4+2kπ