MCQ
સમીકરણ $cos^7x\, +\, sin^4x\, =\, 1$ ના $(-\pi, \pi)$ માં ઉકેલો મેળવો
- A$ - \frac{\pi }{2}\,,\,0$
- B$ - \frac{\pi }{2}\,,\,0\,,\,\frac{\pi }{2}$
- C$ \frac{\pi }{2}\,,\,0$
- D$0\,\,,\,\,\frac{\pi }{4}\,\,,\,\frac{\pi }{2}$
$\cos ^{7} x=1-\sin ^{4} x=\left(1-\sin ^{2} x\right)\left(1+\sin ^{2} x\right)$
$\cos ^{7} x=\cos ^{2} x\left(1+\sin ^{2} x\right)$
$\cos ^{7}(x)-\cos ^{2} x\left(2-\cos ^{2} x\right)=0$
$\cos ^{2} x\left(\cos ^{5} x+\cos ^{2}(x)-2\right)=0$
$\cos ^{2}(x)=0$ implies
$x=\frac{\pm \pi}{2}$
And
$\cos ^{5} x+\cos ^{2} x-2=0$ implies
$\cos (x)=1$
$x=0$
Hence
$x \in\left\{\frac{-\pi}{2}, 0, \frac{\pi}{2}\right\}$
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