# Homework Help: What are the steps to solving this multivariable calculus proof?

1. Sep 11, 2011

### sh3lF1sh

Let a; b and c be any three vectors in R3 with a =6 0. Show that b = c if and only if
a dot b = a dot c and a cross b = a cross c

2. Sep 11, 2011

### lineintegral1

To prove an if and only if statement, you must prove both directions of the theorem. In this case, you must show that $b=c$ implies the dot and cross product equations you provided. You must also show that the dot and cross product equations imply that $b=c$ (which is certainly more challenging, the first direction is relatively trivial).

To prove the latter statement I described, think about the definitions of the cross and dot product. Yes, you've been provided an easy means of calculating them (i.e. $<x,y>\cdot <a,b>=ax+by$), but there is a trigonometric definition for the dot and cross products. Can you manipulate these to show that a=b? Hint: Consider the angle a makes between b and c individually. What must be true about these angles?