i need help

find the real root of the equation z^3 + z + 10=0 given that one root is 1-2i

since 1-2i is a root its complex conjugate 1+2i is also a root

so far i know the sum of the roots is -1

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- Oct 26th 2009, 10:46 AMdeadmotorRoots of polynomial equations
i need help

find the real root of the equation z^3 + z + 10=0 given that one root is 1-2i

since 1-2i is a root its complex conjugate 1+2i is also a root

so far i know the sum of the roots is -1 - Oct 26th 2009, 12:31 PMred_dog
No, the sum of all three roots is 0.

- Oct 26th 2009, 12:44 PMLogic
I came up with an idea for factoring which renders the initial knowledge of any roots unnecessary unless mine idea is faulty.

- Oct 26th 2009, 12:58 PMHallsofIvy
How do you arrive at your very first statement:

if and only if ?

It seems clear to me that z= -2 satisfies the first equation but does NOT satisfy the second. - Oct 26th 2009, 06:39 PMLogic
- Oct 27th 2009, 12:17 AMpacman
deadmotor, you said the 2 roots are 1-2i and 1+2i because they are pair conjugate, but notice the equation z^3 + z + 10 = 0, sum of roots is zero.

if a, b, and c are the roots, then a + b + c = 0,

a + ( 1 + 2i) + (1 - 2i) = 0, surely you know the third root, right? which is a . . . why not evaluate it?

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It can be factored by grouping,

z^3 + z + 10 = 0,

__z^3 +____2z^2____- 2z^2____-____4z__+__4z____+ z + 10__= 0,

z^2(z + 2) - 2z(z + 2) + 5(z + 2) = 0,

(z + 2)(z^2 - 2z + 5) = 0.

surely you know how to use the quadratic formula to find the rest of the roots . . . - Oct 27th 2009, 02:02 AMharbottle
Another possibility: if 1 + 2i and 1 - 2i are roots then

z - (1 + 2i) and z - (1 - 2i) are factors

So (z-1-2i)(z-1+2i) is a factor

Simply this and you will get a quadratic. then you need one more factor, which should now be obvious (there are not many choices now that you have the 2nd power of z).

From there you should get the third root.