A complex root

Algebra Level 4

Find the square root(s) of 2 + i 2+i , (where i {i} = 1 \sqrt{-1} ) in the form x {x} + y {y} i {i} , then give your answer as the value of the expression below.

( x 2 ) × ( y 2 ) \left ( \displaystyle \sum x^2 \right ) \times \left ( \sum y^2 \right )


The answer is 1.

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2 solutions

Eshan Balachandar
Feb 15, 2015

I believe there is much simpler solution. From the equations, we get that roots are z and -z. (x,-x) and (y,-y) Summation x^2 = 2x^2 Summation Y^2 = 2y^2.

Therefore, the question simplifies as 4 x^2 y^2 = 4/4 = 1.

Can you expand on that a little bit more please

Curtis Clement - 6 years, 3 months ago
Curtis Clement
Jan 2, 2015

To solve the square root of 2+ i {i} , firstly you write the square of your hypothetical answer and equate the real and imaginary parts, as follows: ( x + y i ) 2 (x+yi)^2 = x 2 x^{2} - y 2 y^{2} +2 x {x} y {y} i {i} , so x 2 x^{2} - y 2 y^{2} = 2 ( 1) and 2 x {x} y {y} i {i} =1 (2 ). Rearranging (2 ) gives, y {y} = 1 2 x \frac{1}{2x} which we substitute back into equation ( 1): x 2 x^{2} - 1 4 x 2 \frac{1}{4x^2} = 2. Multiply by 4 x 2 4x^{2} and rearranging leads to 4 x 4 x^{4} - 8 x 2 x^{2} -1 = 0. This is a quadratic in disguise, so let t {t} = x 2 x^{2} and solve 4 t 2 t^{2} - 8 t {t} -1 = 0 (and square root the results) to produce the following 2 roots: x 1 x_{1} = 1 + 1 2 5 \sqrt{1+\frac{1}{2}\sqrt{5}} and x 2 x_{2} = 1 2 5 1 \sqrt{\frac{1}{2}\sqrt{5} -1} i {i} . Substitute the value of x 2 x^{2} into equation (*1) yields: y 1 y_{1} = 1 2 5 1 \sqrt{\frac{1}{2}\sqrt{5} -1} and y 2 y_{2} = 1 + 1 2 5 \sqrt{1+\frac{1}{2}\sqrt{5}} i {i} . Now \displaystyle \sum_{}^{} x 2 x^{2} = 2 + 5 \sqrt{5} and \displaystyle \sum_{}^{} y 2 y^{2} = 5 \sqrt{5} -2. The answer is \therefore ( 5 \sqrt{5} +2)( 5 \sqrt{5} -2) = 1

Doesn’t the equation 4x^4-8x^2-1=0 have four solutions? X1, X2 but also X3=-X1 and X4=-X2? If I didn’t mistake the calculations, the sum of x^2 is 4 and the sum of y^2 is -4, so the result is -16

Emanuele Prati - 2 years, 1 month ago

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