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Find a quadratic equation with the roots -1 + 4i and -1 - 4i.

 Dec 16, 2015

Best Answer 

 #2
avatar+2592 
+5

Indeed you did, you had answered before i had even clicked on the question

 Dec 16, 2015
 #1
avatar+8581 
+5

The quadratic expression x2 - 2x + 15 can be factored to give x2 - 2x - 15 = (x - 5)(x + 3). Thus the quadratic equation x2 - 2x + 15 = 0 has roots x = 5 and x = -3. You can perform this from the end to the beginning.

One quadratic equation with roots 5 and -3 is (x - 5)(x + 3) = 0. Expanding this gives x2 - 2x + 15 = 0.

What I am saying is a quadratic equation with roots a and b is (x - a)(x - b) = 0. This fact is true, even ifa and b are complex numbers. Hence one quadratic equation with roots 4 - i and 4 + i is

[x - (4 - i)][x - (4 + i)] = 0.

Expand the left side.

When you are finished you can use the quadratic formula to verify that 4 - i and 4 + 1i are roots.

 

Man, I typed fast!!

 Dec 16, 2015
 #2
avatar+2592 
+5
Best Answer

Indeed you did, you had answered before i had even clicked on the question

SpawnofAngel Dec 16, 2015
 #3
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0

Is that right?

 Dec 16, 2015
 #4
avatar+130466 
+5

Let's expand upon Hayley's answer [it is correct  !!!]

 

[x - (4 - i)][x - (4 + i)] = 0

 

x^2 - x (4 - i) - x (4 + i) +  [4 - i] [ 4 + i]  =

 

x^2 - 4x + ix - 4x - ix  + 16 -4i + 4i - i^2  =

 

x^2 - 8x + 16  + 1  =

 

x^2 - 8x + 17

 

 

 

cool cool cool

 Dec 16, 2015

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