∣∣∣→a+→b∣∣∣2=∣∣→a∣∣2+∣∣∣→b∣∣∣2+2∣∣→a∣∣∣∣∣→b∣∣∣.cosθ
∣∣→a∣∣2+∣∣∣→b∣∣∣2=∣∣→a∣∣2+∣∣∣→b∣∣∣2+2∣∣→a∣∣+∣∣∣→b∣∣∣∵−1⩽cosθ⩽1
Rightarrow2∣∣→a∣∣∣∣∣→b∣∣∣cosθ⩽2∣∣→a∣∣∣∣∣→b∣∣∣
So,
∣∣∣→a+→b∣∣∣2⩽(∣∣→a∣∣∣∣∣→b∣∣∣)2
⇒∣∣∣→a+→b∣∣∣⩽∣∣→a∣∣+∣∣∣→b∣∣∣
Which is the required answer.