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Answer:
a) f(-2.5) = 62.6875; f(-1.5) = -13.0625; (-2.5, -1.5) contains a real root
b) there are 1 each of positive and negative real roots; 2 complex roots
c) -2 and 1/3 are the rational real roots
d) x ∈ {-2, 1/3, -1/3 +i(√2)/3, -1/3 -i(√2)/3}
Step-by-step explanation:
a) f(-2.5) = 62.6875; f(-1.5) = -13.0625. The function changes sign between -1.5 and -2.5, so there is a real root in that interval.
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b) The given coefficients have one sign change, so there is one positive real root. Changing the signs of odd-degree terms makes the signs + - + - -, so there are three sign changes. There are 1 or 3 negative real roots, and possibly 2 complex roots. The constant term is non-zero, so zero is not a root. Here's your table:
positive real roots: 1
negative real roots: 1 or 3
complex roots: 2 or 0
zero roots: none
The graph is attached. It shows 2 real roots, one positive and one negative, so there are 2 complex roots. (There are 4 roots in total, so if only 2 are real, the other 2 are complex.)
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c) Possible rational roots are of the form ...
±{1, 2}/{1, 3, 9}
Listed, they are ...
±{1/9, 2/9, 1/3, 2/3, 1, 2}
The actual real roots are -2 and 1/3.
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d) The graph shows the real roots to be -2 and 1/3. The original leading coefficient of 3 is the product of the leading coefficients of the factors. Writing the factor (x -1/3) as (3x -1) lets us reduce the leading coefficient of the quadratic term from 9 to 3. The result of dividing factors (x+2) and (3x-1) from the original function is shown in the attachment. It is a parabola with vertex (-1/3, 2/3), so the factorization is ...
f(x) = (x +2)(3x -1)(3(x +1/3)² +2/3)
The roots of the quadratic term are ...
3(x +1/3)² = -2/3
x +1/3 = ±√(-2/9)
x = (-1 ±i√2)/3
So, the entire list of roots is ...
x ∈ {-2, 1/3, -1/3 +i(√2)/3, -1/3 -i(√2)/3}
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Additional comment
The roots of a quadratic are easily found by knowing its leading coefficient and its vertex. If factoring known real roots from a higher-degree polynomial results in a quadratic, then the remaining complex roots are easily found. A graphing calculator is a wonderful tool for this process.
