Thank a lot . Is it bco of bisector , we had to reverse it

Bisector means the lines are perpendicular to each other. Is because of the perpendicular (90°), the product of the gradients of these two lines is -1. When you rearrange to find one gradient, it is just "negative reciprocal of the other gradient", as what you meant by "reverse".
Think in your notes, the formula is:
m1*m2=-1, where m1 and m2 are the gradients of the two perpendicular lines respectively.

Thank for the explanation