Question 39 of 96intermediate🔧 ApplyLong Answer4 marks

For the differential equation xydydx=(x+2)(y+2)xy \frac{dy}{dx} = (x + 2) (y + 2), find the solution curve passing through the point (1,−1)(1, -1).

Correct Answer

The given differential equation is:

xydydx=(x+2)(y+2)xy \frac{dy}{dx} = (x + 2) (y + 2)

To solve this, we first separate the variables. Dividing both sides by x(y+2)x(y+2) (where x≠0x \neq 0 and y≠−2y \neq -2), we get:

yy+2dy=x+2xdx\frac{y}{y+2} dy = \frac{x+2}{x} dx

Simplifying the integrands, we write the equation as:

∫(y+2−2y+2)dy=∫(1+2x)dx\int \left( \frac{y+2-2}{y+2} \right) dy = \int \left( 1 + \frac{2}{x} \right) dx ∫(1−2y+2)dy=∫1 dx+2∫1x dx\int \left( 1 - \frac{2}{y+2} \right) dy = \int 1 \, dx + 2 \int \frac{1}{x} \, dx

Integrating both sides, we obtain the general solution:

y−2ln⁡∣y+2∣=x+2ln⁡∣x∣+Cy - 2 \ln|y+2| = x + 2 \ln|x| + C

We need to find the particular solution for the curve passing through the point (1,−1)(1, -1). Substituting x=1x = 1 and y=−1y = -1 into the general solution:

(−1)−2ln⁡∣−1+2∣=1+2ln⁡∣1∣+C(-1) - 2 \ln|-1+2| = 1 + 2 \ln|1| + C −1−2ln⁡(1)=1+0+C-1 - 2 \ln(1) = 1 + 0 + C

Since ln⁡(1)=0\ln(1) = 0, we get −1=1+C-1 = 1 + C, which implies C=−2C = -2.

Substituting the value of C=−2C = -2 back into the general solution, the equation of the required curve is:

y−2ln⁡∣y+2∣=x+2ln⁡∣x∣−2y - 2 \ln|y+2| = x + 2 \ln|x| - 2

Or, rearranging:

y−x+2=2ln⁡∣x(y+2)∣y - x + 2 = 2 \ln|x(y+2)|

∴ The solution curve is y−x+2=2ln⁡∣x(y+2)∣\mathbf{y - x + 2 = 2 \ln|x(y+2)|}.

Exercise: EXERCISE 9.3 | Q: 16 | (Page 12)
For More Understanding

Explanation

The solution follows the method of separation of variables demonstrated in the provided context (specifically Example 7). The context establishes the procedure: separate xx and yy terms, integrate both sides to find a general solution involving a constant CC, and then substitute the given point (1,−1)(1, -1) to find the specific value of CC.

The integration step uses standard logarithmic integrals, consistent with the calculus operations shown in the textbook examples.

Solution Steps

  1. Step 1: Separate variables by dividing both sides by x(y+2)x(y+2) to get:

    yy+2dy=x+2xdx\frac{y}{y+2} dy = \frac{x+2}{x} dx
  2. Step 2: Integrate both sides after simplifying the terms to:

    ∫(1−2y+2)dy=∫(1+2x)dx\int \left( 1 - \frac{2}{y+2} \right) dy = \int \left( 1 + \frac{2}{x} \right) dx
  3. Step 3: Apply the limits of the point (1,−1)(1, -1) to the resulting equation:

    y−2ln⁡∣y+2∣=x+2ln⁡∣x∣+Cy - 2 \ln|y+2| = x + 2 \ln|x| + C

    to find C=−2C = -2.

  4. Step 4: Substitute C=−2C = -2 back into the equation to get the final solution:

    y−2ln⁡∣y+2∣=x+2ln⁡∣x∣−2y - 2 \ln|y+2| = x + 2 \ln|x| - 2