乘坐x(x>3)km
付钱:3+1.2(x-3)=1.2x-0.6(元)
a^2+2ab=-10
b^2+2ab=16
两式相加
(a^2+2ab)+(b^2+2ab)
=a^2+4ab+b^2=(-10)+16=6
两式相减
(a^2+2ab)-(b^2+2ab)
=a^2-b^2=(-10-16)=-26