W=2π/π=2

A=3- 1=2，B= 1

y=2cos(2x+a)+ 1

When x=π/ 12, y= 1.

2 cos(2 *π/ 12+a)+ 1 = 1

cos(π/6+a)=0

π/6+a=π/2

a=π/3

y=2cos(2x+π/3)+ 1