YLL討論網

當A(a,3)與B(-1,b)的連線與直線L：X+2Y=3⊥

試求2a+b=?

由兩線垂直，其斜率乘積 = -1

AB的斜率 = (3-b)/(a+1)

L的斜率 = -1/2

所以[(-1)(3-b)]/[2(a+1)] = -1 => 3-b = 2a+2

=> 2a+b = 1