假设直线是y=kx+b则ak+b=2 (1)5k+b=1 (2)-4k+b=2a (3)(2)-(3)9k=1-2ak=(1-2a)/9(1)-(2)(a-5)k=1所以(a-5)(1-2a)/9=1-2a²+11a-5=92a²-11a+14=0(2a-7)(a-2)=0a=7/2,a=2