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NTA Abhyas 2020
Exam
NTA Abhyas
Year
2020
Subject
Mathematics
Chapter
Differentiation

NTA Abhyas 2020 · Mathematics · Differentiation

If f ( x ) = co s ^ - 1 ( x (4co s ^ 2 x - 1)) , then (1)/(π) f ^ '...

If f(x)=cos−1f ( x ) = co{s}^{- 1}\left(\sin x\left(4co{s}^{2}x - 1\right)\right), then 1πf′(π3)⋅f(π10)\dfrac{1}{\pi}{f}^{'}\left(\dfrac{\pi}{3}\right) ⋅ f\left(\dfrac{\pi}{10}\right) is

Correct answer

0.60

Step-by-step solution

f(x)=cos−1{sinx(4(1−sin2x)−1)}f\left(x\right) = co{s}^{- 1}\left\{sin x\left(4\left(1 - si{n}^{2}x\right) - 1\right)\right\}
f(x)=cos−1{sinx(3−4sin2x)}=cos−1{3sinx−4sin3x}f\left(x\right) = co{s}^{- 1}\left\{sin x\left(3 - 4si{n}^{2}x\right)\right\} = co{s}^{- 1}\left\{3sin x - 4si{n}^{3}x\right\}
f(x)=cos−1(sin⁡3x)=π2−sin−1(sin⁡3x)f\left(x\right) = co{s}^{- 1}\left(\sin 3x\right) = \dfrac{\pi}{2} - si{n}^{- 1}\left(\sin 3x\right)
In the neighbourhood of x=π3⇒f(x)=π2−(π−3x)x = \dfrac{\pi}{3} ⇒ f\left(x\right) = \dfrac{\pi}{2} - \left(\pi - 3x\right)
f(x)=3x−π2f\left(x\right) = 3x - \dfrac{\pi}{2}
∴f′(π3)=3∴ {f}^{'}\left(\dfrac{\pi}{3}\right) = 3
In the neighbourhood of x=π10⇒f(x)=π2−(3x)x = \dfrac{\pi}{10} ⇒ f\left(x\right) = \dfrac{\pi}{2} - \left(3x\right)
f(π10)=π2−3π10=π5f\left(\dfrac{\pi}{10}\right) = \dfrac{\pi}{2} - \dfrac{3\pi}{10} = \dfrac{\pi}{5}
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