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  • Explain solution for RD Sharma maths Class 12 Chapter 21 Differential Equation Exercise Revision Exercise (RE) Question 66 Subquestion (iv) textbook solution.

Explain solution for RD Sharma maths Class 12 Chapter 21 Differential Equation Exercise Revision Exercise (RE) Question 66 Subquestion (iv) textbook solution.

Answers (1)

Answer :  \frac{1}{2}(\sin x-\cos x)+c e^{x}

Hint : You must know the rules of solving differential equation and integration

Given :  \frac{d y}{d x}-y=\cos x

Solution : differential equation is in form of

                 \begin{gathered} \frac{d y}{d x}+P y=Q \\ P=-1 \quad, Q=\cos x \\ \text { I.F }=e^{\int P d x}=e^{\int-1 d x}=e^{-x} \end{gathered}

Solution is

\begin{aligned} &y \times I F=\int Q \times I F d x+c\\ &y e^{-x}=\int e^{-x} \cos x+c\; \; \; \; \; \; \; \; \; \; \; \; \; \; \; ...(i)\\ &\text { let } I=\int e^{-x} \cos x d x \end{aligned}

Integrate by \int f(x) g(x) d x=f(x) \int g(x) d x-\int\left\{f^{\prime}(x) \int g(x) d x\right\} d x

\begin{gathered} \text { Take } f(x)=\cos x \text { and } g(x)=e^{-x} \\ I=\cos x \int e^{-x} d x-\int\left\{(-\sin x) \int e^{-x} d x\right\} d x \\ I=-e^{-x} \cos x-\int-\sin x\left(-e^{-x}\right) d x \end{gathered}

\begin{aligned} &I=-e^{-x} \cos x-\int e^{-x} \sin x d x \\ &I=-e^{-x} \cos x-\left\{\sin x \int e^{-x} d x-\int\left(\cos x \int e^{-x} d x\right) d x\right\} \end{aligned}

\begin{aligned} &I=-e^{-x} \cos x-\left(-e^{-x} \sin x\right)+\int-e^{-x} \cos x d x \\ &I=-e^{-x} \cos x+e^{-x} \sin x-\int e^{-x} \cos x d x \\ &I=e^{-x}(\sin x-\cos x)-I \\ &2 I=e^{-x}(\sin x-\cos x) \\ &I=\frac{e^{-x}}{2}(\sin x-\cos x) \end{aligned}

From (i)

\begin{aligned} &y e^{-x}=\int e^{-x} \cos x+c \\ &y e^{-x}=\frac{e^{-x}}{2}(\sin x-\cos x)+c \\ &y=\frac{1}{2}(\sin x-\cos x)+c e^{x} \end{aligned}

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