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4-2.Quadratic Equations and Inequations
hard
समीकरण $x ^7-7 x -2=0$ के विभिन्न वास्तविक मूलों की संख्या होगी
A
$5$
B
$7$
C
$1$
D
$3$
(JEE MAIN-2022)
Solution
$x^{7}-7 x-2=0$
$x^{7}-7 x=2$
$f(x)=x^{7}-7 x \text { (odd) } and \,y=2$
$f(x)=x\left(x^{2}-7^{1 / 3}\right)\left(x^{4}+x^{2} \cdot 7^{1 / 3}+7^{2 / 3}\right)$
$f^{\prime}(x)=7\left(x^{6}-1\right)=7\left(x^{2}-1\right)\left(x^{4}+x^{2}+1\right)$
$f^{\prime}(x)=0 \Rightarrow x=\pm 1$
$f(x)=2$ has $3$ real distinct solution
Standard 11
Mathematics