A new inverse trigonometric function question

Calculus Level 3

Let M M be the greatest and m m be the least value of sin 1 x + cos 1 x \sqrt{\sin^{-1}x}+\sqrt{\cos^{-1}x} , then find the value of ( M m ) 2 \left(\dfrac{M}{m}\right)^2


The answer is 2.

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1 solution

Tom Engelsman
Apr 18, 2021

Taking the first derivative equal to zero yields:

f ( x ) = 1 2 1 x 2 [ 1 arcsin ( x ) 1 arccos ( x ) ] = 0 \Large f'(x) = \frac{1}{2\sqrt{1-x^2}} \cdot [ \frac{1}{\sqrt{\arcsin(x)}} - \frac{1}{\sqrt{\arccos(x)}}] = 0 ;

or arcsin ( x ) = arccos ( x ) arcsin ( x ) = arcsin ( 1 x 2 ) x = 1 x 2 2 x 2 = 1 x = 1 2 \arcsin(x) = \arccos(x) \Rightarrow \arcsin(x) = \arcsin(\sqrt{1-x^2}) \Rightarrow x = \sqrt{1-x^2} \Rightarrow 2x^2 = 1 \Rightarrow x = \frac{1}{\sqrt{2}}

The second derivative at x = 1 2 x = \frac{1}{\sqrt{2}} yields:

f ( 1 / 2 ) = 8 π 3 / 2 < 0 f''(1/\sqrt{2}) = -\frac{8}{\pi^{3/2}} < 0 (a maximum). Hence, M = π / 4 + π / 4 = 2 π / 2 = π M = \sqrt{\pi/4} + \sqrt{\pi/4} = 2 \cdot \sqrt{\pi}/2 = \sqrt{\pi} .

The minimum value, m , m, occurs when arcsin ( x ) = 1 , arccos ( x ) = 0 x = 1 \arcsin(x) = 1, \arccos(x)= 0 \Rightarrow x = 1 , or π / 2 + 0 = π / 2 \sqrt{\pi/2} + \sqrt{0} = \sqrt{\pi/2} . Finally, ( M m ) 2 = ( π / ( π / 2 ) ) 2 = ( 2 ) 2 = 2 . (\frac{M}{m})^2 = (\sqrt{\pi/(\pi/2)})^2 = (\sqrt{2})^2 = \boxed{2}.

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