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Montrer que \(\dfrac{\pi}4 = 4\arctan (\dfrac 15) - \arctan (\dfrac 1{239})\) (on montrera que \(0 \leq \arctan (\dfrac 15) \leq \dfrac{\pi}8\) et \(0 \leq \arctan (\dfrac 1{239}) \leq \dfrac{\pi}2\)).
Montrer que \(\dfrac{\pi}4 = 4\arctan (\dfrac 15) - \arctan (\dfrac 1{239})\) (on montrera que \(0 \leq \arctan (\dfrac 15) \leq \dfrac{\pi}8\) et \(0 \leq \arctan (\dfrac 1{239}) \leq \dfrac{\pi}2\)).