Prove that : `|veca + vecb|le|veca|+|vecb|`.

Explanation

Let ABC be the triangle where `vec(AB) = veca. vec(BC) = vecb`
Then `vec(AC) = veca=vecb`
Now `|veca| = AB, |vecb| = BC` and `|veca+vecb| = AC`
In the triangle ABC,
`AC `implies |veca+vecb|<|veca|+|vecb|`
Now AC = AB+BC when A,B,C lie on a straight line A,B,C are collinear.
Hence `|veca+vecb| = |veca|+|vecb|`
When `veca` and `vecb` are like collinear vectors or zero vectors.