It is User:wonghang.

Pi formula proofs edit

Proof of Leibniz's formula edit

Machin's formula

{\frac {\pi }{4}}=4\tan ^{-1}{\frac {1}{5}}-\tan ^{-1}{\frac {1}{239}}

Recall the formulas:

\tan(x+y)={\frac {\tan x+\tan y}{1-\tan x\tan y}}

\tan(x-y)={\frac {\tan x-\tan y}{1+\tan x\tan y}}

\tan(2x)={\frac {2\tan x}{1-\tan ^{2}x}}

Let

\tan \alpha ={\frac {1}{5}}

We can obtain tan(2α) = 5/12 and tan(4α) = 120/119 by using the above formula. Therefore,

\tan(4\tan ^{-1}{\frac {1}{5}})={\frac {120}{119}}

Consider,

\tan(4\tan ^{-1}{\frac {1}{5}}-\tan ^{-1}{\frac {1}{239}})={\frac {{\frac {120}{119}}-{\frac {1}{239}}}{1+{\frac {120}{119}}{\frac {1}{239}}}}={\frac {120\cdot 239-119}{119\cdot 239+120}}={\frac {120(120+119)-119}{119(120+119)+120}}={\frac {120^{2}+119^{2}}{120^{2}+119^{2}}}=1

4\tan ^{-1}{\frac {1}{5}}-\tan ^{-1}{\frac {1}{239}}={\frac {\pi }{4}}