The curve passing through the point(1,2) that cuts each member of the family of parabolas y2=4ax orthogonally is :
A
2x2+y2=6
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B
x2+y2=5
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C
x2+2y2=9
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D
y2−x2=3
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Solution
The correct option is A2x2+y2=6 y2=4ax..(1) ⇒dydx=2ay=2ayy2=2ay4ax=y2x=m Now equation of orthogonal curve of (1) is dydx=−1/m=−2xy ⇒ydy+2xdx=0 Integrating we get ⇒y2+2x2=c2 where c2 is any constant Now given this curve is passing through (1,2) ⇒22+2(1)2=c2⇒c2=6 Hence equation of required curve is 2x2+y2=6