计算：（x+1)分之1+2
=1+2(x+1)/(x+1)
=(2x+3)/(x+1)
计算：2x分之y+（3x的平方）分之y
=(3xy+2y^2)/(6x^2)
计算：（a的平方-4)分之（a-6)+（a-2)分之1
=((a-6)+(a+2))/(a^2-4)
=(2a-4)/(a+2)(a-2)
=2(a-2)/(a+2)(a-2)
=2/(a+2)