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Question

Equation of common tangent of y=x2,y=−x2+4x−4 is

A
y=4(x1)
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B
y=0
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C
y=4(x1)
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D
y=30x50
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Solution

The correct options are
A y=4(x1)
B y=0
The equation of tangent to y=x2, be y=mxm24. Putting in y=x2+4x4, we should only get one value of x i.e. Discriminant must be zero.
mxm24=x2+4x4
x2+x(m4)+4m24=0
D=0
Now, (m4)2(16m2)=0
2m(m4)=0m=0,4
y=0 and y=4(x1) are the required tangents.
Hence, (a) and (b) are correct answers.

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