Inspiring

Algebra Level 5

If z z is a complex numbers that satisfies z 3 + 2 z + z = 68994 + 40 i , |z|^3+2|z|+z=68994+40i, what is the absolute value of 3 times the real part of z z added to 4?


The answer is 31.

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2 solutions

Josh Banister
Feb 7, 2015

From the expression, z 3 |z|^3 is real, 2 z 2|z| is real and z z is complex. As the complex part of the RHS = complex part of the LHS, i m g ( z 3 + 2 z + z ) = i m g ( z ) = i m g ( 68994 + 40 i ) = 40 i m g ( z ) = 40 img(|z|^3 + 2|z| + z) = img(z) = img(68994 + 40i) = 40 \\ img(z) = 40

Now to find the real part, let z = x + 40 i z = x + 40i such that z = x 2 + 4 0 2 = x 2 + 1600 |z| = \sqrt{x^2 + 40^2} = \sqrt{x^2 + 1600} . Using z 3 = z z 2 |z|^3 = |z||z|^2 , The given equation now becomes x 2 + 1600 ( x 2 + 1600 ) + 2 x 2 + 1600 + x = 68994 x 2 + 1600 ( x 2 + 1602 ) + x = 68994 \sqrt{x^2 + 1600}(x^2 + 1600) + 2\sqrt{x^2 + 1600} + x = 68994 \\ \sqrt{x^2 + 1600}(x^2 + 1602) + x = 68994

By trying out numbers (which was influenced by trying to make the surds into integers) I noticed by letting x = ± 9 x = \pm 9 , the surd terms became ( ± 9 ) 2 + 1600 = 1681 = 41 \sqrt{(\pm9)^2 + 1600} = \sqrt{1681} = 41 so by substituting x = ± 9 x = \pm9 into the equation, the the result was 41 1683 ± 9 = 68994 41 * 1683 \pm 9 = 68994 which is true for the negative value of ± 9 \pm9 so x = 9 x = -9 is a solution and by extension, z = 9 + 40 i z = -9 + 40i

The final solution is now 3 9 + 4 = 31 3|\mathord{-} 9| + 4 = \boxed{31}

Hey!! Why didn't you get any upvotes ? Well , +1 from me :)

A Former Brilliant Member - 6 years, 2 months ago
Parveen Soni
Nov 22, 2014

let z=a+ib and |z|=(a^2+b^2)^(1/2) so comparing real and img. part we have b=40 and thus (a^2+1600)^(3/2)+2((a^2+1600)^(1/2))+a=68994 on solving this we have a=-9 so answer=3x|-9|+4=31

can you use latex sir.

Mardokay Mosazghi - 6 years, 6 months ago

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