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Determine the value of life g(x)=4x+k is a tangent to f(x)=-x^2+8x+20​

Sagot :

Answer:

k=16

Step-by-step explanation:

So the tangent line is

[tex]4x + k[/tex]

and it tangent to function

[tex] {x}^{2} + 8x + 20[/tex]

Since the slope of the tangent line is 4, this means the derivative of f(x) is 4 but first let find the derivative of

[tex] {x}^{2} + 8x + 20[/tex]

Use the Sum Rule,

[tex] \frac{d}{dx} {x}^{2} + \frac{d}{dx} 8x + \frac{d}{dx} 20[/tex]

Use the Power Rule and we get

[tex]2x + 8[/tex]

Set this equal to 4

[tex]2x + 8 = 4[/tex]

[tex]2x = - 4[/tex]

[tex]x = - 2[/tex]

So at x=-2, the slope of the tangent line is 4.

Plug -2 in the orginal function, and we get

[tex] { - 2}^{2} + 8( - 2) + 20 = 8[/tex]

So the point must pass through -2,8 with a slope of 4.

[tex]y - 8 = 4(x + 2)[/tex]

[tex]y - 8 = 4x + 8[/tex]

[tex]y = 4x + 16[/tex]

So the value of k is 16.