An algebra problem by me...

Algebra Level 3

a + b + c = 1 a + b + c=1 , a 2 + b 2 + c 2 = 9 a^{2} + b^{2} + c^{2}=9 , a 3 + b 3 + c 3 = 1 a^{3} + b^{3} + c^{3}=1 , then find 1 a + 1 b + 1 c \frac{1}{a} +\frac{1}{b} + \frac{1}{c}


The answer is 1.

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3 solutions

Sarthak Rath
Feb 22, 2015

a 2 + b 2 + c 2 = ( a + b + c ) 2 2 ( a b + b c + c a ) = 9 a^{2}+b^{2}+c^{2} = (a+b+c)^2 - 2(ab+bc+ca)=9

p u t t i n g putting v a l u e s values i n in t h i s , this,

1 2 ( a b + b c + c a ) = 9 1 - 2(ab+bc+ca) = 9

a b + b c + c a = 4 ( 1 ) ab+bc+ca = -4------------(1)

n o w now

( a 2 + b 2 + c 2 ) ( a + b + c ) = a 3 + b 3 + c 3 + a b ( a + b ) + b c ( b + c ) + (a^{2}+b^{2}+c^{2})(a+b+c) = a^{3}+b^{3}+c^{3} + ab(a+b) + bc(b+c) +

c a ( c + a ) = 9 ca(c+a) = 9

1 + a b ( 1 c ) + b c ( 1 a ) + c a ( 1 b ) = 9 1 + ab(1-c) + bc(1-a) + ca(1-b) = 9

a b + b c + c a 3 a b c = 8 ab + bc + ca - 3abc = 8

a b c = 4 ( 2 ) abc = -4----------------(2)

m o d i f y i n g modifying t h e the e x p r e s s i o n expression w e we n e e d need t o to f i n d , find,

1 a + 1 b + 1 c = a b + b c + c a a b c \frac{1}{a} + \frac{1}{b} + \frac{1}{c} = \frac{ab + bc + ca}{abc}

p u t t i n g putting ( 1 ) (1) a n d and ( 2 ) (2) , w e we g e t get

4 4 = 1 \frac{-4}{-4} = \boxed{1}

Vishal S
Feb 25, 2015

1 a \frac {1}{a} + 1 b \frac {1}{b} + 1 c \frac {1}{c}

\Rightarrow a b + b c + c a a b c \frac {ab+bc+ca}{abc} ---->(1)

We know that

( a + b + c ) 2 (a+b+c)^2 = a 2 a^2 + b 2 b^2 + c 2 c^2 +2(ab+bc+ca)

By substituting the given values in the above identity, we get

2(ab+bc+ca)=1-9

\Rightarrow ab+bc+ca=-4---->(2)

We know that

a 3 a^3 + b 3 b^3 + c 3 c^3 =(a+b+c)( a 2 a^2 + b 2 b^2 + c 2 c^2 -(ab+bc+ca))+3abc

By substituting the given values and (1) in the above identity, we get

1=(1)(9-(-4))+3abc

\Rightarrow 3abc=-12

\Rightarrow abc=-4---->(3)

(1)= ( 2 ) ( 3 ) \frac {(2)}{(3)}

\Rightarrow ( 4 ) ( 4 ) \frac {(-4)}{(-4)} = 1 \boxed{1}

Lalit Jena
Feb 22, 2015

(a+b+c)^2=a^2+b^2+c^2+2ab+2bc+2ca So value of ab+bc+ca is -4

And from a^3+b^3+c^3=(a+b+c+)(a^2+b^2+c^3) + 3abc And value of abc come -4 1/a+1/b+1/c= (ab+bc+ca)/abc =-4/-4 U gotta answer and I gotta nothing :-( Nice question Sarthak ghoda gadi.......:-P

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