Domains!

Geometry Level 4

Let us say that the domain of the function f ( x ) = log sin x + cos x ( 2 sin x ) f(x)=\sqrt{\log_{\large{\sin x+\cos x}} {(2\sin x})} where 0 x π 0 \leq x \leq \pi is of the form [ a , b ) [a,b) , what is 24 a b \dfrac{24a}{b} ?


The answer is 8.

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1 solution

Sabhrant Sachan
Jun 22, 2016
  • Base For a function g ( x ) = log a b a ( 0 , 1 ) ( 1 , ) In this case : sin x + cos x = 2 sin ( x + π 4 ) ( 0 , 1 ) ( 1 , 2 ] Since sin x is +ve in x [ 0 , π ] sin ( x + π 4 ) is +ve in x [ 0 , 3 π 4 ] Also , 2 sin ( x + π 4 ) 1 x 0 , π 2 For our base : x ( 0 , π 2 ) ( π 2 , 3 π 4 ) \quad \text{Base} \\ \quad \text{For a function } g(x) = \log_{a}{b} \\ \quad a \in (0,1) \cup (1,\infty) \\ \quad \text{In this case : } \\ \quad \sin{x}+\cos{x} = \sqrt{2} \sin{\left( x + \dfrac{\pi}{4} \right) } \in (0,1) \cup \left(1,\sqrt2 \right] \\ \quad \text{Since } \sin{x} \text{ is +ve in } x \in \left[0,\pi\right] \\ \quad \sin{\left( x + \dfrac{\pi}{4} \right) } \text{ is +ve in } x \in \left[0,\dfrac{3\pi}{4}\right] \\ \quad \text{Also , } \sqrt{2} \sin{\left( x + \dfrac{\pi}{4} \right) } \ne 1 \\ \quad x \ne 0,\dfrac{\pi}{2} \\ \quad \text{For our base : } \boxed{x \in \left(0,\dfrac{\pi}{2} \right) \cup \left(\dfrac{\pi}{2},\dfrac{3\pi}{4} \right)} \\ \\

  • 2 2 sin x > 0 1 sin x > 0 x ( 0 , π ) \quad 2 \ge 2\sin{x}>0 \implies 1 \ge \sin{x}>0 \\ \quad \boxed{x \in \left( 0 , \pi \right)} \\ \\

  • Condition : log 2 sin ( x + π 4 ) 2 sin x 0 log 2 sin ( x + π 4 ) 2 sin x log 2 sin ( x + π 4 ) 1 For x ( 0 , π 2 ) , 2 sin x 1 x [ π 6 , 5 π 6 ] x [ π 6 , π 2 ) For x ( π 2 , 3 π 4 ) , 2 sin x 1 x ( 0 , π 6 ] [ 5 π 6 , π ) x ϕ \quad \text{Condition : } \log_{\sqrt{2} \sin{\left( x + \frac{\pi}{4} \right) }}{2\sin{x}} \ge 0 \\ \quad \log_{\sqrt{2} \sin{\left( x + \frac{\pi}{4} \right) }}{2\sin{x}} \ge \log_{\sqrt{2} \sin{\left( x + \frac{\pi}{4} \right) }}{1} \\ \quad \text{For } x \in \left(0,\dfrac{\pi}{2} \right) \quad , \quad 2\sin{x} \ge 1 \\ \quad x \in \left[ \dfrac{\pi}{6} , \dfrac{5\pi}{6} \right] \\ \implies \boxed{ x \in \left[ \dfrac{\pi}{6} , \dfrac{\pi}{2} \right) } \\ \quad \text{For } x \in \left(\dfrac{\pi}{2},\dfrac{3\pi}{4} \right) \quad , \quad 2\sin{x} \le 1 \\ \quad x \in \left( 0 , \dfrac{\pi}{6} \right] \cup \left[ \dfrac{5\pi}{6} , \pi \right) \\ \implies \boxed{ x \in \phi } \\ \\

Our answer : x [ π 6 , π 2 ) a = π 6 , b = π 2 24 × π 6 π 2 8 \quad \text{Our answer : } \boxed{ x \in \left[ \dfrac{\pi}{6} , \dfrac{\pi}{2} \right) } \implies a= \dfrac{\pi}{6} , b=\dfrac{\pi}{2} \\ \quad \dfrac{24 \times \dfrac{\cancel{\pi}}{6}}{\dfrac{\cancel{\pi}}{2}} \implies \boxed{8}


Nice Question , Keep posting \text{Nice Question , Keep posting }

Brilliant question as well as solution! :)

Prakhar Bindal - 4 years, 11 months ago

Exact same solution!Very good!It wasn't my own :P though!But if i find any good ones,i will surely post!

Adarsh Kumar - 4 years, 11 months ago

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