If two sides of a triangle are `hati+2hatj and hati+hatk`, then find the length of the third side.

Explanation

Given sides of the triangle are `veca=hati+2hatj and vecb=hati+hatk`.
If vector along the third side is `vecc`, then we must have `veca+vecb+vecc=0`. Then
`" "vecc=-(hati+2hatj)-(hati+hatk)=-2hati-2hatj-hatk`
Therefore, the length of the third side `|vecc|` is `sqrt((-2)^(2)+(-2)^(2)+(-1)^(2)) =3`.