Chapter 1.7, Problem 40ES

Use the result in Exercise 39 to show that

(a) ${\tan }^{-1}\frac{1}{2}+{\tan }^{-1}\frac{1}{3}=\pi /4$

(b) $2{\tan }^{-1}\frac{1}{3}+{\tan }^{-1}\frac{1}{7}=\pi /4$