根据定义:a☆b=axb+a+b,对于方程1/2x☆3=5
1/2 x ☆ 3 = 1/2 x x 3 + 1/2 x + 3 = 5
即:3/2 x + 1/2x = 2
2x = 2
解得: x = 1