Does this property hold for inverse too) (Part-V)

Calculus Level 4

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

Determine the number of real solution of x x which satisfy the equation above.

Original problem.
1 i n f t y infty 2 0 3

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1 solution

Yashas Ravi
Dec 19, 2020

Let arctan x = ß \arctan{x} = ß . If we draw a right triangle where the side opposite to ß ß is a a and the side adjacent to ß ß is b b , then x = tan ( ß ) = a b x = \tan(ß) = \frac{a}{b} . Now, ß = 1/arccot(x) = 1/arccot(a/b) If we take cot ( ( π / 2 ) ß ) \cot((π/2)-ß) , we get a b \frac{a}{b} . Thus, arccot(a/b) = (π/2)-ß. By substitution, ß = 1 ( ( π / 2 ) ß ) ß = \frac{1}{((π/2)-ß)} , meaning that ß 2 + ( π / 2 ) ß 1 = 0 -ß^2 + (π/2)ß - 1 = 0 . The discriminant comes out to be ( π 2 / 4 ) 1 < 0 (π^2/4) - 1 < 0 , meaning that there are no values of ß ß satisfying the original equation. No calculus requried!

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