a*b=a+2b
——
2b
将a=3、b=4代入计算就行了.
3+2×3
3*4* 2分之1 = ————* 1/2
2×3
9
= —* 1/2
6
= 3/2* 1/2
3/2+2×1/2
= ——————
2×1/2
=5/2