Maths-
General
Easy

Question

Tangents are drawn from the point (-1,2) on the parabola y to the power of 2 end exponent equals 4 x The length, these tangents will intercept on the line x equals 2:

  1. 6    
  2. 6 square root of 2    
  3. 2 square root of 6    
  4. none of these    

hintHint:

 

The correct answer is: 6 square root of 2


    The length of tangent is asked to be find
    if the line x=2 intersects these tangents at (x1,y1) and (x2,y2) then the length of the intercept is given by |y1y2|
    SS1=T2 is the equation of pair of tangents
    (y24x)(8)=4(yx+1)2
    y22y(1x)(x2+6x+1)=0
    Put x=2
    y2+2y17=0
    Clearly, y1,y2 are the roots of the equation y2+2y17=0
    y1+y2=2
    y1y2=17
    Now, |y1y2|=(y1+y2)24y1y2

    =224(17)
    =72
    =62
    |y1y2|=62

    Hence finally the length is 6 square root of 2

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