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Algebra Level 3

{ a 2 + b 2 + c 2 = 18 a b + b c + c a = 9 \large \begin{cases} a^2 + b^2\ +c^2 =18 \\ ab+bc+ca =9 \end{cases}

If a a , b b , and c c satisfy the system of equations above, what is the value of a + b + c |a|+|b|+|c| ?

It cannot be determined 6 6

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2 solutions

I will share a solution soon, but here are two examples of possible values for a , b , c a,b,c that satisfy the system of equations above , but a + b + c |a|+|b|+|c| is not the same , watch out that a , b , c a,b,c can be complex

if a , b , c a,b,c are a = 0 , b = 3 , c = 3 a=0,b=3,c=3 these values satisfy the system of equations above and |a|+|b|+|c|=6

if a , b , c a,b,c are a = 10 , b = 2 i 45 , c = 2 + i 45 a=10,b=-2-i\sqrt{45},c=-2+i\sqrt{45} these values satisfy the system of equations above and |a|+|b|+|c|=24

Richard Costen
Feb 25, 2019

( a + b + c ) 2 = a 2 + b 2 + c 2 + 2 ( a + b + c ) (a+b+c)^2=a^2+b^2+c^2+2(a+b+c) ( a + b + c ) 2 = 18 + 2 ( 9 ) = 36 (a+b+c)^2=18+2(9)=36 a + b + c = ± 6 a+b+c=\pm 6 a + b + c = 6 |a|+|b|+|c|=6 only if a , b , c 0 a,b,c \ge 0 or a , b , c 0 a,b,c \le 0 . For example, a = 2 , b = 7 , c = 3 a + b + c = 6 a=2,b=7,c=-3\implies a+b+c=6 but a + b + c = 12 |a|+|b|+|c|=12

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