Region of complex numbers!

Algebra Level 5

Consider the region S S of complex number a a such that z 2 + a z + 1 = 1 |z^2+az+1|=1 , where complex number z z is on the locus z = 1 |z|=1 . If A A denotes the area of S S in the Argand plane, find A 2 \left \lfloor \frac A2 \right \rfloor .

Notations:


The answer is 5.

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2 solutions

Guilherme Niedu
Dec 6, 2016

Let us denote z z as c o s ( θ ) + j s i n ( θ ) cos(\theta) + j\cdot sin(\theta) and a a as x + j y x + jy

z 2 + a z + 1 = 1 | z^2 + az + 1| = 1

c o s ( 2 θ ) + j s i n ( 2 θ ) + ( x c o s ( θ ) y s i n ( θ ) ) + j ( x s i n ( θ ) + y c o s ( θ ) ) + 1 = 1 | cos(2\theta) + jsin(2\theta) + (xcos(\theta) - ysin(\theta)) + j(xsin(\theta) +ycos(\theta)) + 1| = 1

( c o s ( 2 θ ) + ( x c o s ( θ ) y s i n ( θ ) ) + 1 ) 2 + ( x s i n ( θ ) + y c o s ( θ ) + s i n ( 2 θ ) ) 2 = 1 (cos(2\theta) + (xcos(\theta) - ysin(\theta)) + 1)^2 + (xsin(\theta) +ycos(\theta) + sin(2\theta))^2 = 1

( 2 c o s 2 ( θ ) + x c o s ( θ ) y s i n ( θ ) ) 2 + ( x s i n ( θ ) + y c o s ( θ ) + 2 s i n ( θ ) c o s ( θ ) ) 2 = 1 (2cos^2(\theta) + xcos(\theta) - ysin(\theta) )^2 + (xsin(\theta) +ycos(\theta) + 2sin(\theta)cos(\theta))^2 = 1

x 2 + y 2 + 4 c o s 2 ( θ ) + 4 x c o s ( θ ) = 1 x^2 + y^2 + 4cos^2(\theta) + 4xcos(\theta) = 1

( x + 2 c o s ( θ ) ) 2 + y 2 = 1 (x+2cos(\theta))^2 + y^2 = 1

Since 2 c o s ( θ ) 2cos(\theta) varies from 2 -2 to 2 2 , the figure is equivalent to a circle with center with y y coordinate equal to 0 0 and x x coordinate varying from 2 -2 to 2 2 , which means a figure like:

Which means that the area will be a rectangle of base 4 4 and height 2 2 plus 2 semi-circles (1 circle) of radius equal to 1 1 . Or:

A = 4 2 + π 1 2 A = 4\cdot 2 + \pi\cdot 1^2

A = 8 + π = 11.1416... A = 8 + \pi = 11.1416...

Then A 2 = 5 \color{#3D99F6} \large \lfloor \frac{A}{2} \rfloor = \fbox{5}

U can also simplify like this

z = 1 z 2 = 1 |z|=1 \implies |z|^2=1 .Now z 2 + a z + 1 = 1 z 2 + a z + z 2 = 1 z 2 + a z + z z = 1 z z + z + a = 1 |z^2+az+1|=1 \\ \implies |z^2 + az +|z|^2|=1 \\ \implies |z^2 +az +z \overline{z}|=1 \\ \implies |z||z +\overline{z} +a|=1 .

We can cosider z = cos θ + i sin θ z=\cos \theta +i \sin \theta so z + z = 2 cos θ z+ \overline{z}=2 \cos \theta .

2 cos θ + a = 1 |2 \cos \theta +a|=1 and then ur solution.

Kushal Bose - 4 years, 6 months ago

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yes .exactly the way i approached .

avi solanki - 4 years, 6 months ago

Whoa!! Nice approach! I did it the long way

Sumanth R Hegde - 4 years, 2 months ago

How you got this figure.. ?

Ayaen Shukla - 3 years, 5 months ago

It seems that this question would have n n solvers n n likes

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