Show that the magnitude of vector sum of `vec(a) and vec(b)` cannot be greater than `|vec(a)| + | vec(b)|` or smaller than `|vec(a)|- |vec(b)|`

Explanation

The resultant of two vectors `vec(a) and vec(b)` acting at angle `theta ` , is
`R = sqrt(a^(2) +b^(2)+2ab cos theta)`
When `theta = 0 " "R_(max) = sqrt(a^(2)+b^(2)+2ab) = a+b`
When `theta = 0" " R_(max) =sqrt(a^(2)+b^(2)-2ab)=a-b`