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Question

A straight line parallel to the line 2x−y+5=0 is also a tangent to the curve y2=4x+5. Then, the point of contact is

A
(2,1)
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B
(−1,1)
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C
(1,3)
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D
(3,4)
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E
(−1,2)
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Solution

The correct option is C (−1,1)
Given curve: y2=4x+5
Equation of line parallel to 2x−y+5=0 is 2x−y+k=0. where, k is a constant.
Slope of the line =2 ...... (1)
Slope of tangent to the curve is
2ydydx=4⇒dydx=2y ......(2)
From (1) and (2), we get
2y=2
⇒y=1
Putting value y=1 in equation of curve, we get
12=4x+5⇒4x=−4
⇒x=−1
Point of contact=(−1,1)
Hence, B is the correct option.

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