Question

F(x)=2x^3-3x^2-12x+k

a)show that if k=30, then f(x)=0 has at least one solution
b)show that if k=30, then f(x)=0 has exactly one solution
c)find all values of k so that f(x)=0 has exactly one solution

Answer 1

I'm going to replace f(x) with y since we're talking about graphing here.

dy/dx = 12 - 6x - 6x^2..................Set this = 0

-6x^2 - 6x + 12 = 0....................Divide both sides by (-6)

x^2 + x - 2 = 0

(x + 2) * (x - 1) = 0

x = 1, -2

Take the second derivative: d^2y/dx^2 = -6 - 12x

This is negative at x = 1 and positive at x = -2 so we have a maximum at x = 1 and a relative minimum at x = -2

The function is increasing on (-2, 1) <------------ Answer for (a)
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f(x)= k +12x - 3x^2 - 2x^3

f(1) = k + 12 - 3 - 2 = k + 7

If you want that to be 4, then k + 7 = 4 and k = - 3 <-------------- Answer for k---(b)
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I guess that I should be considering k to be -3 from here on out. You weren't specific about whether I should leave it as k or change it to -3.

So from now on the equation is: f(x)= -2x^3 -3x^2 + 12x - 3 <--------------
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f(-2) = 16 -12 -24 - 3 = 16 - 39 = - 23

So the minimum is (-2, -23) <------------------- Answer to (c)

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(-∞,-2) <------------ Answer to (d)