Exercise 1.2.6. (a) Verify the triangle inequality in the special case where $a$ and $b$ have the same sign. (6) Find an efficient proof for all the cases at once by first demonstrating $Catb)^{2} \leq lal+[6]){2}$. c) Prove $ja-b| \leqa-c|+|c-d[+|d-b|$ for all $a, b, c$, and $0$. (d) Prove, 11$al-1 bil Vlela-b|$. (The unremarkable identity $a-a-b+b$ may be useful.) SP.SD. 346