The Minimum Complex ?

Algebra Level 4

If z z is any complex number satisfying z 3 2 i 2 |z-3-2i|\leq 2 then find the maximum value of 2 z 6 + 5 i |2z-6+5i| .


The answer is 13.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Chew-Seong Cheong
Nov 14, 2016

z 3 2 i 2 Let z = x + y i x 3 + ( y 2 ) i 2 ( x 3 ) 2 + ( y 2 ) 2 4 \begin{aligned} |z-3-2i| & \le 2 & \small {\color{#3D99F6}\text{Let }z = x+yi} \\ |x-3+(y-2)i| & \le 2 \\ (x-3)^2 + (y-2)^2 & \le 4 \end{aligned}

We note that y y is maximum when x = 3 x=3 . That is ( y 2 ) 2 = 4 (y-2)^2 = 4 y m a x = 4 \implies y_{max} = 4 .

Now, consider

2 z 6 + 5 i = 2 z 3 + 2.5 i = 2 ( x 3 ) 2 + ( y + 2.5 ) 2 = 2 ( x 3 ) 2 + ( y 2 ) 2 ( y 2 ) 2 + ( y + 2.5 ) 2 2 4 ( y 2 ) 2 + ( y + 2.5 ) 2 2 4 y 2 + 4 y 4 + y 2 + 5 y + 6.25 2 9 y + 6.25 2 9 y m a x + 6.25 2 42.25 13 \begin{aligned} |2z-6+5i| & = 2|z-3+2.5i| \\ & = 2\sqrt{(x-3)^2 + (y+2.5)^2} \\ & = 2\sqrt{(x-3)^2 + (y-2)^2 - (y-2)^2 + (y+2.5)^2} \\ & \le 2\sqrt{4 - (y-2)^2 + (y+2.5)^2} \\ & \le 2\sqrt{4 - y^2+4y-4 + y^2+5y+6.25} \\ & \le 2\sqrt{9y+6.25} \\ & \le 2\sqrt{9y_{max}+6.25} \\ & \le 2\sqrt{42.25} \\ & \le \boxed{13} \end{aligned}

Kushal Bose
Nov 12, 2016

2 z 6 + 5 i = 2 z 6 4 i + 9 i 2 z 6 4 i + 9 i = 2 z 3 2 i + 9 i 2.2 + 9 = 13 |2z-6+5i|=|2z-6-4i+9i| \leq |2z-6-4i| + |9i|= 2|z-3-2i| +|9i| \leq 2.2+9=13

For two complex numbers z 1 , z 2 z_1,z_2 we know that z 1 + z 2 z 1 + z 2 |z_1|+|z_2| \geq|z_1+z_2|

The minimum value of z 3 2 i = 2 |z-3-2i|=2 which is given

The question asks for minimum value, and not maximum. I think the minimum value is 5. @Md Zuhair @Kushal Bose can you recheck?

Siva Bathula - 4 years, 7 months ago

Log in to reply

Can u post ur solution

Kushal Bose - 4 years, 7 months ago

Log in to reply

Ok the question has been changed to finding 'maximum' now. Well I used lagrangian multipliers with inequality constraint.

Siva Bathula - 4 years, 7 months ago

I ask the moderators to change the answer as per Kushal Bose

Md Zuhair - 4 years, 7 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...