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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: short-cut for finding square -root of a complex no. in MCQs
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sourab_MCA (7)

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can ne 1 give me pointers on how to know the square root of complex no. just by looking at the
given options.(no (a+ib)^2=x+iy, or formula method).
for ex. sq root of
7-24i
is
a)3+2i,3-2i
b)4-3i,-4+3i
c)4-3i,4+3i
d)3-4i,3+4i


    
RyuAmakusa (451)

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take half of y and split as 2 no.s a,b such that a^2 - b^2 = x

eg. 7 - 24i. half of 24=12 = 4+3.

4^2 - 3^2 = 7 so the ans is (+or -) 4-3i
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Conjurer (512)

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The formula is +-root[(r+a)/2] +/-(sign of b)root[(r-a)/2]i
where r = |a+ib|

In this case r = 25, a =7 and sign of b = -

So roots are +-root[(25 + 7)/2] - root[(25-7)/2]i = +- (4 - 3i)

Let us learn to dream, gentlemen, and then perhaps we shall learn the truth.
- August Kekule
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sboosy (2970)

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\mbox{If the required square root is} \ x+iy \\ \\ \mbox{then} \ x^2+y^2 = \mbox{Hypotenuese of 7 and 24} = 25 \\ \\ x^2-y^2 = 7\ \mbox{Real part} \\ \\ \Rightarrow x=\pm4 ,y=\pm3 \\ \\ \mbox{Sq. root is} \ \pm(x+\mbox{(Coeff of imag part)}iy) \\ \\ \mbox{In the question coeff of imag part ..that is 24 is negative.Hence} \\ \\ \mbox{Sq.root is} \ \pm(4-3i)
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budokai_tenkaichi (62)

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7-4i = (a+bi)2
7-24i = a2-b2+2abi
a2-b2=7             ab=-12   
b=-12/a
a2-144/a2=7
a4-144=7a2
a4-7a2-144=0
a4-16a2+9a2-144=0
a2(a2-16)+9(a2-16)=0
(a2+9)(a2-16)=0
a2=-9(not happening)
a=+4 or -4
b=-3 or +3
 
root is (4-3i) or (3-4i)
  




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budokai_tenkaichi (62)

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well i should say 'conjurer' ji has a very good method

may i ask for the verification of this method (rather prove it,.,.,.,.,.,.,.,.,.,.)

Vegeta: While I was with you guys, I became more human. I didn't like that. I even have a family and started to like living on earth.
Vegeta: Don't remind me. I'm mad enough to hurt somebody and pounding you just might be the therapy I need.
Vegeta: You can take control of my mind and my body, but there is one thing a Saiyan always keep... his PRIDE!
[Piccolo and Vegeta sit back to back on a tiny island]
Piccolo: Is it over?
Vegeta: Not until the fish jumps.
[a fish jumps out of the water]
Vegeta: It's over.
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Conjurer (512)

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Straight out of the book :D



Let us learn to dream, gentlemen, and then perhaps we shall learn the truth.
- August Kekule
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budokai_tenkaichi (62)

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gee  thanxx

Vegeta: While I was with you guys, I became more human. I didn't like that. I even have a family and started to like living on earth.
Vegeta: Don't remind me. I'm mad enough to hurt somebody and pounding you just might be the therapy I need.
Vegeta: You can take control of my mind and my body, but there is one thing a Saiyan always keep... his PRIDE!
[Piccolo and Vegeta sit back to back on a tiny island]
Piccolo: Is it over?
Vegeta: Not until the fish jumps.
[a fish jumps out of the water]
Vegeta: It's over.
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budokai_tenkaichi (62)

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amazng man..

Vegeta: While I was with you guys, I became more human. I didn't like that. I even have a family and started to like living on earth.
Vegeta: Don't remind me. I'm mad enough to hurt somebody and pounding you just might be the therapy I need.
Vegeta: You can take control of my mind and my body, but there is one thing a Saiyan always keep... his PRIDE!
[Piccolo and Vegeta sit back to back on a tiny island]
Piccolo: Is it over?
Vegeta: Not until the fish jumps.
[a fish jumps out of the water]
Vegeta: It's over.
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sourab_MCA (7)

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what i was looking for was something like ryuamakasa gave.
thnx mate. :)

as i mentioned , i already knew other methods described here
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cuteprincess (17)

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pls solve:
find sq.rt of -7+24i by a short cut method
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cuteprincess (17)

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the ans is (3+4i) and (-3-4i)
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sourab_MCA (7)

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using the formula(when b>0)


+/- {( [(|z|+a)/2]+i [(|z|-a)/2]}

=   +/- { 18/2 + i 32/2}
=   +/-(3+4i)


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