SOLUTION: If -2 is a root of ((z^3) - (8(z^2)) + (9z) + 58 = 0, what are the other two roots?

Algebra ->  Complex Numbers Imaginary Numbers Solvers and Lesson -> SOLUTION: If -2 is a root of ((z^3) - (8(z^2)) + (9z) + 58 = 0, what are the other two roots?      Log On


   



Question 1181497: If -2 is a root of ((z^3) - (8(z^2)) + (9z) + 58 = 0, what are the other two roots?
Answer by ikleyn(52754) About Me  (Show Source):
You can put this solution on YOUR website!
.
If -2 is a root of ((z^3) - (8(z^2)) + (9z) + 58 = 0, what are the other two roots?
~~~~~~~~~~~~~~~~~

In fact, -2 is a root of the given polynomial, and you can check it in one line, 
substituting the value of -2 into the polynomial

    %28-2%29%5E3 - 8%2A%28-2%29%5E2 + 9%2A%28-2%29 + 58 = -8 - 32 - 18 + 58 = 0.



Therefore, the given polynomial is divisible by (z-2) without a remainer (according to the Remainder theorem)

and you can divide it using standard long division procedure 


    %28z%5E3+-+8z%5E2+%2B+9z+%2B+58%29%2F%28z-2%29 = z^2 - 10z + 29.


Solve this quadratic equation using the Quadratic Formula


    z%5B1%2C2%5D = %2810+%2B-+sqrt%2810%5E2+-+4%2A29%29%29%2F2 = %2810+%2B-+sqrt%28-16%29%29%2F2 = %2810+%2B-+4i%29%2F2 = 5 +- 2i.


ANSWER.  Two other roots of the given equation are  (5+2i)  and  (5-2i).

Solved.