0 & lt= x & lt=π/2

-2π/3 & lt； =π/3-2x & lt； =π/3

The graph of -cos is inverted,-1

When a<0, 2a < =-2acos <; =-2a，3a+b & lt； = f(x)& lt； =b

3a+b=-5 and b= 1 Look for A, and B looks for A.

When a=0. . . . . . . . . . (Write it yourself, similar to the above)

When a>0 ................................. (write it yourself, similar to the above)