Medium INTEGER +4 / -1 PYQ · JEE Mains 2021

Let the domain of the function

$$f(x) = {\log _4}\left( {{{\log }_5}\left( {{{\log }_3}(18x - {x^2} - 77)} \right)} \right)$$ be (a, b). Then the value of the integral $$\int\limits_a^b {{{{{\sin }^3}x} \over {({{\sin }^3}x + {{\sin }^3}(a + b - x)}}} dx$$ is equal to _____________.

Answer (integer) 1

Solution

For domain<br><br>${\log _5}\left( {{{\log }_3}(18x - {x^2} - 77)} \right) &gt; 0$<br><br>${\log _3}(18x - {x^2} - 77) &gt; 1$<br><br>$18x - {x^2} - 77 &gt; 3$<br><br>${x^2} - 18x + 80 &lt; 0$<br><br>$x \in (8,10)$<br><br>$\Rightarrow$ a = 8 and b = 10<br><br>$$I = \int\limits_a^b {{{{{\sin }^3}x} \over {{{\sin }^3}x + {{\sin }^3}(a + b - x)}}} dx$$<br><br>$$I = \int\limits_a^b {{{{{\sin }^3}x(a + b - x)} \over {{{\sin }^3}x + {{\sin }^3}(a + b - x)}}} $$<br><br>$2I = (b - a) \Rightarrow I = {{b - a} \over 2}$ ($\because$ a = 8 and b = 10)<br><br>$I = {{10 - 8} \over 2} = 1$

About this question

Subject: Mathematics · Chapter: Definite Integration · Topic: Properties of Definite Integrals

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