Question 41 of 96intermediate🔧 ApplyLong Answer4 marks

At any point (x,y)(x, y) of a curve, the slope of the tangent is twice the slope of the line segment joining the point of contact to the point (−4,−3)(-4, -3). Find the equation of the curve given that it passes through (−2,1)(-2, 1).

Correct Answer

The equation of the curve is y=(x+4)2−3y = (x + 4)^2 - 3 or y=x2+8x+13y = x^2 + 8x + 13.

Exercise: EXERCISE 9.3 | Q: 18 | (Page 12)
For More Understanding

Explanation

This is a differential equations problem from the NCERT textbook. The key is to first express the slope of the line segment joining the point of contact (x,y)(x, y) to the point (−4,−3)(-4, -3), then set up the differential equation using the given condition that the tangent's slope is twice this line segment's slope.

The differential equation formed is separable and can be solved by integration. Finally, the particular solution is found by substituting the given point (−2,1)(-2, 1).

Solution Steps

  1. Step 1: Find the slope of the line segment joining point (x,y)(x, y) to point (−4,−3)(-4, -3).

    slope=y−(−3)x−(−4)=y+3x+4\text{slope} = \frac{y - (-3)}{x - (-4)} = \frac{y + 3}{x + 4}
  2. Step 2: Set up the differential equation. Since slope of tangent is twice the slope of line segment:

    dydx=2×y+3x+4\frac{dy}{dx} = 2 \times \frac{y + 3}{x + 4}
  3. Step 3: Separate variables and integrate. Rearranging:

    dyy+3=2 dxx+4\frac{dy}{y + 3} = \frac{2\,dx}{x + 4}

    Integrating both sides:

    ln⁡∣y+3∣=2ln⁡∣x+4∣+C1\ln|y + 3| = 2\ln|x + 4| + C_1
  4. Step 4: Simplify the equation. This gives:

    y+3=C(x+4)2y + 3 = C(x + 4)^2

    where C=eC1C = e^{C_1}.

  5. Step 5: Find constant C using given condition. Since curve passes through (−2,1)(-2, 1):

    1+3=C(−2+4)21 + 3 = C(-2 + 4)^2 4=C(4)4 = C(4) C=1C = 1
  6. Step 6: Write the final equation. Substituting C=1C = 1:

    y+3=(x+4)2y + 3 = (x + 4)^2

    ∴ y=x2+8x+13\mathbf{y = x^2 + 8x + 13}