JEE Advance - Mathematics (2015 - Paper 2 Offline - No. 16)
Let $$f'\left( x \right) = {{192{x^3}} \over {2 + {{\sin }^4}\,\pi x}}$$ for all $$x \in R\,\,$$ with $$f\left( {{1 \over 2}} \right) = 0$$.
If $$m \le \int\limits_{1/2}^1 {f\left( x \right)dx \le M,} $$ then the possible values of $$m$$ and $$M$$ are
If $$m \le \int\limits_{1/2}^1 {f\left( x \right)dx \le M,} $$ then the possible values of $$m$$ and $$M$$ are
$$m=13,$$ $$M=24$$
$$\,m = {1 \over 4},M = {1 \over 2}$$
$$m=-11,$$ $$M=0$$
$$m=1,$$ $$M=12$$
Explanation
We have, $$f'(x) = {{192{x^3}} \over {2 + {{\sin }^4}\pi x}}$$
Given, $$f\left( {{1 \over 2}} \right) = 0$$ and $$m \le \int\limits_{1/2}^1 {f(x)dx \le m} $$
$$\therefore$$ f(x) is increasing in $$\left( {{1 \over 2},1} \right)$$.
$$\therefore$$ $$f'{(x)_{\max }} = {{192} \over {24}} = 96$$
$$ \Rightarrow 96 = {{f(1) - f(1/2)} \over {1/2}} \Rightarrow f(1) = 96 \times {1 \over 2} = 48$$
$$M = {1 \over 2} \times {1 \over 2} \times 48 = 12$$
$$f'{(x)_{\min .}} = {{\left( {{{192} \over 8}} \right)} \over 3} = {{192} \over {24}}$$
$$ \Rightarrow {{192} \over {24}} = {{f(1) - 0} \over {(1/2)}} \Rightarrow f(1) = {{192} \over {24}} \times {1 \over 2}$$
$$\therefore$$ $$m = {1 \over 2} \times {1 \over 2} \times {{192} \over {24}} \times {1 \over 2} = 1$$
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