- #1

madah12

- 326

- 1

## Homework Statement

A= ci-2j+k

B=i+2j-k

Find c that makes the vector(A-B) perpendicular to the vector B

## Homework Equations

AXB = (AxBy-AyBx)k+(AzBx-AxBz)j+(AyBz-AzBy)i

## The Attempt at a Solution

A-B=(c-1)i-4j+2k

I said that since (A-B) is perpendicular to B then

|A-B| * |B| = |AXB| because sine 90 =1

then I got

5((c-1)^2+20)=5(c+1)^2

which gave me a solution of C=5

but

then A dot B didnt turn out to be zero why?