20 13 the process of filling in the fifteenth question of science mathematics in Fujian college entrance examination.
This is to add CN (1) and CN (2) to the first expression in turn. . . Cn(n), {only take n terms}, then sum to the power of (1+x), and then integrate the upper limit of 1/2 and the lower limit of 0 on both sides, and finally multiply by 1/2.