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

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