B. f is always negative on the interval
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Explanation:
-1/5 = -0.2
Pick any number for x that will make the interval -0.2 < x < 2 to be true. I find x = 0 to be easiest.
Plug it into f(x)
f(x) = (5x+1)(4x-8)(x+6)
f(0) = (5(0)+1)(4(0)-8)(0+6)
f(0) = (1)(-8)(6)
f(0) = -48
We get a negative result.
So we can rule out choice A which says that f is always positive. Either f is always negative on this interval, or it's a mix of being positive and negative.
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Note that the roots of f(x) are -1/5, 2 and -6. This is from solving f(x) = 0
Use the zero product property to solve (5x+1)(4x-8)(x+6) to find the three roots mentioned.
The roots of -1/5 and 2 form the boundary of the interval mentioned at the top of the problem. There are no roots in between -1/5 and 2, so this means that f(x) stays entirely negative on this interval. There is no way f(x) becomes positive on this interval because it would have to cross over the x axis, thus forming another root. But again there are no roots between -1/5 and 2.
A graph confirms we have the correct answer. Check out the image attached below. Note the portion from x = -0.2 to x = 2 is entirely below the x axis.
