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CBSE Class 12 Maths 2010 Solved Paper

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Question : 20 of 29
Marks: +1, -0
Find the equations of the normals to the curve y = x3 + 2x + 6 which are parallel to the line x + 14y + 4 = 0.
Solution:  
Equation of the curve is y = x3x^3 + 2x + 6
Slope of the normal at point (x, y) = 1dydx\frac{-1}{\frac{dy}{dx}}
dydx\frac{dy}{dx} = 3x23x^2 + 2
On substitution, we get
Slope of the normal = 13x2+2\frac{-1}{3x^2+2} ... (1)
Normal to the curve is parallel to the line x + 14y + 4 = 0,
i.e. y = - 114×414\frac{1}{14} \times \frac{-4}{14}
So the slope of the line is the slope of the normal.
Slope of the line is - 114\frac{1}{14} = 13x2+2\frac{-1}{3x^2+2}
3x23x^2 + 2 = 14
3x23x^2 = 12
x2x^2 = 4
⇒ x = ± 2
When x = 2, y = 18 and when x = - 2, y = - 6
Therefore, there are two normals to the curve y = x3x^3 + 2x + 6.
Equation of normal through point (2, 18) is given by:
y - 18 = 114\frac{-1}{14} (x - 2)
⇒ 14y - 252 = - x + 2
⇒ x + 14y - 254 = 0
Equation of normal through point (-2,-6) is given by:
y - (- 6) = 114\frac{-1}{14} (x - (- 2))
⇒ 14y + 84 = - x - 2
⇒ x + 14y + 86 = 0
Therefore, the equation of normals to the curve are x + 14y – 254 = 0 and x + 14y + 86 = 0
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