The equation of the line is y=b-4x with slope = -4

The parabola has slope 2ax and at x=4 this is 8a

Therefore 8a=-4 and a = -1/2.

But the tangent and the line must share the same point, namely (4,-8) (from y=(-1/2)x^{2})

Therefore -8=b-4*4 and b=8

Ahmad A.

asked • 04/14/20
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The equation of the line is y=b-4x with slope = -4

The parabola has slope 2ax and at x=4 this is 8a

Therefore 8a=-4 and a = -1/2.

But the tangent and the line must share the same point, namely (4,-8) (from y=(-1/2)x^{2})

Therefore -8=b-4*4 and b=8

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