An algebra problem by Ahmad Hesham

Algebra Level 4

. . . . . .

0 Or 3 3 0 0 Or 1

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1 solution

Ahmad Hesham
Jul 24, 2014

x = 1 + ω x + ω 2 x ( 1 ) B y m u t i p l y i n g b y ω x : x ω x = ω x + ω 2 x + ω 3 x ω 3 x = 1 x ω x = ω x + ω 2 x + 1 ( 2 ) F r o m ( 1 ) , ( 2 ) : x = x ω x x x ω x = 0 x ( 1 ω x ) = 0 x = 0 O r ω x = 1 x = 3 N o w t h e r e s s o m e t h i n g I m i s s e d w h i c h o n e o f t h e s o l v e r s n o t i f y m e a b o u t : ) I f y o u t r i e d i t , y o u d f i n d o u t t h a t { x = 0 } d o e s n t s a t i s f y t h e e q u a t i o n b e c a u s e i f y o u s u b s t i t u t e x w i t h 0 y o u l l h a v e : 0 = 1 + 1 + 1 0 = 3 w h i c h i s r e j e c t e d o f c o u r s e S o w e d c o m e u p w i t h x = 3 a s a f i n a l s o l u t i o n x\quad =\quad 1\quad +\quad { \omega }^{ x }\quad +{ \omega }^{ 2x }\quad \rightarrow \quad (1)\\ \\ By\quad mutiplying\quad by\quad { \omega }^{ x }\quad :\\ \\ x{ \omega }^{ x }\quad =\quad { \omega }^{ x }\quad +\quad { \omega }^{ 2x }\quad +\quad { \omega }^{ 3x }\\ \\ \because { \omega }^{ 3x }=1\\ \\ \therefore x{ \omega }^{ x }\quad =\quad { \omega }^{ x }\quad +\quad { \omega }^{ 2x }\quad +\quad 1\quad \rightarrow \quad (2)\\ \\ From\quad (1)\quad ,\quad (2)\quad :\\ \\ x\quad =\quad x{ \omega }^{ x }\\ \\ x\quad -\quad x{ \omega }^{ x }\quad =\quad 0\\ \\ x(1\quad -\quad { \omega }^{ x })\quad =\quad 0\\ \\ \therefore \quad x\quad =\quad 0\quad \quad Or\quad { \omega }^{ x }\quad =\quad 1\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \therefore \quad x\quad =\quad 3\\ \\ Now\quad there's\quad something\quad I\quad missed\quad which\quad one\quad of\quad \\ the\quad solvers\quad notify\quad me\quad about\quad :)\\ \quad \quad \\ \quad *If\quad you\quad tried\quad it\quad ,\quad you'd\quad find\quad out\quad that\quad \{ x=0\} \quad \\ doesn't\quad satisfy\quad the\quad equation\quad \\ because\quad if\quad you\quad substitute\quad x\quad with\quad 0\quad you'll\quad have\quad :\\ 0=1+1+1\\ 0=3\\ which\quad is\quad rejected\quad of\quad course\\ \\ So\quad we'd\quad come\quad up\quad with\quad \boxed { x=3 } \quad as\quad a\quad final\quad solution

how come substituting 0 to original equation does not preserve the equality? A complex number raised to 0 is 1 right? So wouldn't substituting x with 0 equal to:

0 = 1 + 1 + 1 0 = 1 + 1 + 1

0 = 3 0 = 3

Is there something wrong with my logic?

DPK ­ - 6 years, 10 months ago

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Thanks. I have updated the answer to 3 only.

The mistake made in @Ahmad Hesham 's solution is that he initially multiplied by x x , which introduced the extraneous solution x = 0 x = 0 .

Calvin Lin Staff - 6 years, 10 months ago

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Well , what I really did was multiplying by ω x { \omega }^{ x } However, the solution x = 0 x=0 actually doesn't satisfy the equation and it's my fault , sorry about it

Ahmad Hesham - 6 years, 10 months ago

Hello from my knowledge at the point where we get the solutions as 0 or 3, we have to choose which is applicable. Usually while solving a quadratic we used to get multiple answers but then we prioritize those and select the actual answer. So I think the answer is only 3 because substituting 0 is not making any sense.

Mithun K - 6 years, 10 months ago

Totally missed it , sorry guys !!

I'll edit it right now !

Ahmad Hesham - 6 years, 10 months ago

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FYI - To type equations in Latex, you just need to add \ (   \ )  around your math code, as opposed to all of the text. In this way, you don't have to use \quad all the time.

Calvin Lin Staff - 6 years, 10 months ago

EXTREMELY bad choice of options.

See that x = 0 x=0 does NOT satisfy the condition, because it will lead to 0 = 3 0=3 , impossible.

Only 1 option does NOT contain 0 -_-

Aditya Raut - 6 years, 8 months ago

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