Maths-
General
Easy

Question

The equation of the tangent to the curve x2 + 2y = 8 which is the perpendicular to x – 2y + 1 = 0 is

  1. 2x + y – 6 = 0    
  2. 2x + y – 5 = 0    
  3. 2x – y – 7 = 0    
  4. 2x + y = 0    

hintHint:

We are given the equation of curve. We have to find the equation of tangent perpendicular to the given line. It means the the slope of tangent will be negative reciprocal of slope of given line. We will first find the slope of the line. Using the slope, we will find the equation of tangent.

The correct answer is: 2x + y – 6 = 0


    The given equation of curve x2 + 2y = 8
    The equation of given line is x - 2y + 1 = 0
    x space minus space 2 y space plus space 1 space equals 0
1 space minus space 2 fraction numerator d y over denominator d x end fraction space equals 0
space space space space space space space space 2 fraction numerator d y over denominator d x end fraction equals 1
space space space space space space space space space space space fraction numerator d y over denominator d x end fraction equals 1 half
    This is the slope of the line.
    We will find the slope of the tangent. The tangent is perpendicular to the line. So, we can write
    S l o p e space o f space tan g e n t space equals negative fraction numerator 1 over denominator S l o p e space o f space l i n e end fraction
space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space equals space minus fraction numerator 1 over denominator begin display style bevelled 1 half end style end fraction
space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space equals space minus 2
    Now, we will differentiate the equation of curve to get the slope of tangent.
    x squared space plus space 2 y space equals space 8
2 x space plus space 2 fraction numerator d y over denominator d x end fraction equals 0
2 fraction numerator d y over denominator d x end fraction equals negative 2 x
space space space fraction numerator d y over denominator d x end fraction equals negative x
space space space space
    The equation of tangent of line passing through points (a,b) is
    N o w comma space fraction numerator d y over denominator d x space end fraction space g i v e s space u s space s l o p e space o f t h e space tan g e n t.
W e space a l r e a d y space h a v e space t h e space v a l u e space
s l o p e space equals space minus 2 space
S o comma
space space fraction numerator d y over denominator d x end fraction equals negative x
space minus 2 space equals space minus x

x space equals space 2
    We will put this value in the equation of the curve to find the value of y. 
    x2 + 2y = 8
    (2)2 + 2y = 8
    2y = 8 - 4
    2y = 4
    y = 2
    So, the points at which we have tangent is (2,2)
    Now, we will find the equation of the tangent. If a tangent is at point (x1,y1) to the curve the equation is
    (y - y1) = slope(x - x1)
    (y - 2) = -2(x - 2)
    y - 2 = -2x + 4
    2x + y - 6 = 0
    So, the equation of tangent is 2x + y - 6

    For such questions, we should know the formula to find the tangent and slope of lines and curves.

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