Question

If f(x)=x^3-9x^2+11x+21f(x)=x 3 −9x 2 +11x+21 and x-3x−3 is a factor of f(x)f(x), then find all of the zeros of f(x)f(x) algebraically.

Answer 1

x = -1, 3, and 7

Explanation:

Using grouping:

f(x) = x³ − 9x² + 11x + 21

f(x) = x³ − 9x² + 18x − 7x + 21

f(x) = x (x² − 9x + 18) − 7 (x − 3)

f(x) = x (x − 6) (x − 3) − 7 (x − 3)

f(x) = (x² − 6x) (x − 3) − 7 (x − 3)

f(x) = (x² − 6x − 7) (x − 3)

f(x) = (x + 1) (x − 7) (x − 3)

To use long division, see the picture.

The zeros are x = -1, 3, and 7.