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Algebra Level 4

z 10 z 5 992 = 0 \large { z }^{ 10 }-{ z }^{ 5 }-992=0

What is the number of roots of the equation above for which its real part is negative?


The answer is 5.

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2 solutions

Ravi Dwivedi
Jul 5, 2015

Moderator note:

Good job with chasing down all of the roots.

Sorry its |z|=2 in the first line......

Ravi Dwivedi - 5 years, 11 months ago
Ayush Verma
Jul 6, 2015

z 5 = 1 ± 1 + 4 × 992 2 = 32 , 31 { z }^{ 5 }=\cfrac { 1\pm \sqrt { 1+4\times 992 } }{ 2 } =32,-31

Now we will use only common sense,

L e t z 5 = 32 h a v e m r o o t f o r w h i c h R e ( z ) < 0 n o . o f r o o t s o f z 5 = 31 f o r w h i c h R e ( z ) > 0 = m ( w h y ? ) n o . o f r o o t s o f z 5 = 31 f o r w h i c h R e ( z ) < 0 = ( 5 m ) T o t a l n u m b e r r o o t s f o r w h i c h R e ( z ) < 0 = m + ( 5 m ) = 5 Let\quad { z }^{ 5 }=32\quad have\quad m\quad root\quad for\quad which\quad Re\left( z \right) <0\\ \\ \Rightarrow no.\quad of\quad roots\quad of\quad { z }^{ 5 }=-31\quad for\quad which\quad Re\left( z \right) >0\quad \\ \\ =m\quad \left( why? \right) \\ \\ \Rightarrow no.\quad of\quad roots\quad of\quad { z }^{ 5 }=-31\quad for\quad which\quad Re\left( z \right) <0\quad \\ \\ =\left( 5-m \right) \\ \\ \therefore Total\quad number\quad roots\quad for\quad which\quad Re\left( z \right) <0\\ \\ =m+(5-m)=5\\

Note-This method worked only because there is no possible root for which Re(z)=0

Re(z) is real part of z . ( x of x+iy )

The "Note" is indeed an important part. E.g. It would not have worked for x 12 x 6 992 x^{12} - x^6 - 992 .

Calvin Lin Staff - 5 years, 11 months ago

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