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NDA 2020
Exam
NDA
Year
2020
Subject
Mathematics
Chapter
Differentiation

NDA 2020 · Mathematics · Differentiation

If e^ θ =c+4 θ , where c is an arbitrary constant and is a function...

If eθφ=c+4θφe^{\theta \varphi}=c+4 \theta \varphi, where cc is an arbitrary constant and φ\varphi is a function of θ\theta, then what is φdθ\varphi \mathrm{d} \theta equal to?

Options

  1. Aθdφ\theta \mathrm{d} \varphi
  2. B−θdφ-\theta \mathrm{d} \varphi
  3. C4θdφ4 \theta \mathrm{ d} \varphi
  4. D−4θdφ-4 \theta \mathrm{ d} \varphi

Correct answer

B. −θdφ-\theta \mathrm{d} \varphi

Step-by-step solution

eθϕ=C+4θ⋅ϕ ( θ⋅ϕ)ln⁡e=ln⁡(C+4θ⋅ϕ)θ⋅ϕ=ln⁡(C+4θ⋅ϕ)⇒θ⋅gϕ=4{ln⁡(θ)+ln⁡(ϕ)}+ln⁡C\begin{aligned} & e^{\theta \phi}=C+4 \theta \cdot \phi \\ & \text{ ( } \theta \cdot \phi) \ln e=\ln (C+4 \theta \cdot \phi) \\ & \theta \cdot \phi=\ln (C+4 \theta \cdot \phi) \\ & \Rightarrow \theta \cdot g \phi=4\{\ln (\theta)+\ln (\phi)\}+\ln C \end{aligned} Differentiating both sides with respect to ' θ\theta '. ϕ+θ⋅dϕdθ=4(1θ+1ϕ⋅dϕdθ)ϕ⋅dθ+θ⋅dϕ=4ϕ⋅dθ+4⋅θ⋅dϕ\begin{aligned} & \phi+\theta \cdot \frac{d \phi}{d \theta}=4\left(\frac{1}{\theta}+\frac{1}{\phi} \cdot \frac{d \phi}{d \theta}\right) \\ & \phi \cdot d \theta+\theta \cdot d \phi=4 \phi \cdot d \theta+4 \cdot \theta \cdot d \phi\end{aligned}⇒3θ⋅dϕ=−3ϕ⋅dθ∴ϕ⋅dθ=−θ⋅dϕ\begin{aligned} & \Rightarrow 3 \theta \cdot d \phi=-3 \phi \cdot d \theta \\ & \therefore \phi \cdot d \theta=-\theta \cdot d \phi\end{aligned}$
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