# if vectors a and b are two unit vectors and theta be the angle between them, find the value of theta such that a+b is a unit vector.

$\\\vec a,\vec b are unit vectors \\\vec c=\vec a+\vec b \\|\vec c|=|\vec a+\vec b| \\=>|\vec c|^2=|\vec a+\vec b|^2 \\=>1=1+1+2cos(\theta) \\=>cos(\theta)=\frac{-1}{2} \\=>\theta =120^0$