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The curve having differential equation, ...

The curve having differential equation, `xcosy(dy)/(dx)+siny=x` and passing through the origin, also passes through

A

`(2,(pi)/(2))`

B

`(-2, (pi)/(2))`

C

`(4, (3pi)/(2))`

D

`(-8, (3pi)/(2))`

Text Solution

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The correct Answer is:
To solve the differential equation \( x \cos y \frac{dy}{dx} + \sin y = x \) and find the curve that passes through the origin, we will follow these steps: ### Step 1: Rewrite the Differential Equation We start with the given differential equation: \[ x \cos y \frac{dy}{dx} + \sin y = x \] We can rearrange it to isolate \(\frac{dy}{dx}\): \[ x \cos y \frac{dy}{dx} = x - \sin y \] \[ \frac{dy}{dx} = \frac{x - \sin y}{x \cos y} \] ### Step 2: Separate Variables Now, we can separate the variables \(y\) and \(x\): \[ \frac{dy}{x - \sin y} = \frac{dx}{x \cos y} \] ### Step 3: Integrate Both Sides Next, we integrate both sides. The left side requires integrating with respect to \(y\) and the right side with respect to \(x\): \[ \int \frac{dy}{x - \sin y} = \int \frac{dx}{x \cos y} \] This integration can be complex, but we can use the product rule for differentiation to simplify our work. ### Step 4: Use Product Rule Using the product rule, we recognize that: \[ \frac{d}{dx}(x \sin y) = x \frac{dy}{dx} + \sin y \] Thus, we can express our equation as: \[ \frac{d}{dx}(x \sin y) = x \] ### Step 5: Integrate the Product Rule Result Now we can integrate: \[ \int \frac{d}{dx}(x \sin y) = \int x \, dx \] This gives us: \[ x \sin y = \frac{x^2}{2} + C \] ### Step 6: Find the Constant \(C\) Since the curve passes through the origin \((0, 0)\), we substitute \(x = 0\) and \(y = 0\): \[ 0 \cdot \sin(0) = \frac{0^2}{2} + C \implies 0 = 0 + C \implies C = 0 \] ### Step 7: Write the Particular Solution Thus, the particular solution is: \[ x \sin y = \frac{x^2}{2} \] or \[ \sin y = \frac{x}{2} \] ### Step 8: Check Points Now we will check which points the curve passes through by substituting the \(x\) values from the options into the equation \(\sin y = \frac{x}{2}\). 1. **Option 1: (2, \(\frac{\pi}{2}\))** \[ \sin y = \frac{2}{2} = 1 \implies y = \frac{\pi}{2} \quad \text{(Correct)} \] 2. **Option 2: (-2, \(\frac{\pi}{2}\))** \[ \sin y = \frac{-2}{2} = -1 \implies y = -\frac{\pi}{2} \quad \text{(Incorrect)} \] 3. **Option 3: (4, y)** \[ \sin y = \frac{4}{2} = 2 \quad \text{(Not possible, as sin cannot exceed 1)} \] 4. **Option 4: (-8, y)** \[ \sin y = \frac{-8}{2} = -4 \quad \text{(Not possible, as sin cannot be less than -1)} \] ### Conclusion The only point that the curve passes through, in addition to the origin, is: \[ \boxed{(2, \frac{\pi}{2})} \]
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