(2) When X belongs to [0, pai/2], 2x+pai/6 belongs to [pai/6, 7pai/6]. Combined with the image of sinx, 1+a+2=4, so a= 1.

(3) According to the first two questions, y=2sin(pai/6+2x)+2, so f(x) is obtained by sinx changing the period into pai, then shifting the unit length of pai/6 to the left, and finally doubling it up and down, and then shifting it up by one unit length.