Quadratic equation

Algebra Level 2

Which of the following is a quadratic equation whose roots are 1 2 ( 2 ± 3 i ) \dfrac{1}{2}(2 \pm 3i) ?

4 x 2 8 x + 13 = 0 4x^2-8x+13=0 x 2 8 x + 4 = 0 x^2-8x+4=0 x 2 4 x + 9 = 0 x^2-4x+9=0 2 x 2 + 8 x + 13 = 0 2x^2+8x+13=0

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2 solutions

Chew-Seong Cheong
Sep 28, 2017

The quadratic equation is given by:

( x 2 + 3 i 2 ) ( x 2 3 i 2 ) = 0 x 2 ( 2 + 3 i 2 + 2 3 i 2 ) x + ( 2 + 3 i 2 ) ( 2 3 i 2 ) = 0 x 2 2 x + 13 4 = 0 4 x 2 8 x + 13 = 0 \begin{aligned} \left(x - \frac {2+3i}2\right) \left(x - \frac {2-3i}2\right) & = 0 \\ x^2 - \left(\frac {2+3i}2 + \frac {2-3i}2\right)x + \left(\frac {2+3i}2\right)\left(\frac {2-3i}2\right) & = 0 \\ x^2 - 2x + \frac {13}4 & = 0 \\ \implies 4x^2 - 8x + 13 & = 0 \end{aligned}

[ x 1 2 ( 2 + 3 i ) ] [ x 1 2 ( 2 3 i ) ] = 0 [x-\dfrac{1}{2}(2+3i)][x-\dfrac{1}{2}(2-3i)]=0

( x 2 2 3 i 2 ) ( x 2 2 + 3 i 2 ) = 0 (x-\dfrac{2}{2}-\dfrac{3i}{2})(x-\dfrac{2}{2}+\dfrac{3i}{2})=0

( x 1 3 i 2 ) ( x 1 + 3 i 2 ) = 0 (x-1-\dfrac{3i}{2})(x-1+\dfrac{3i}{2})=0

Multiplying both sides by 2 2 , we get

( 2 x 2 3 i ) ( 2 x 2 + 3 i ) = 0 (2x-2-3i)(2x-2+3i)=0

[ ( 2 x 2 ) 3 i ] [ ( 2 x 2 ) + 3 i ] = 0 [(2x-2)-3i][(2x-2)+3i]=0

( 2 x 2 ) 2 9 i 2 = 0 (2x-2)^2-9i^2=0

but: i 2 = 1 i^2=-1

4 x 2 8 x + 4 9 ( 1 ) = 0 4x^2-8x+4-9(-1)=0

4 x 2 8 x + 4 + 9 4x^2-8x+4+9

4 x 2 8 x + 13 = 0 \color{#D61F06}\boxed{4x^2-8x+13=0}

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