Solving equations

Algebra Level 3

{ 9 a 2 24 b = 17 16 b 2 + 4 c = 7 4 c 2 6 a = 1 \large \begin{cases} 9a^2-24b=-17\\ 16b^2+4c=7\\ 4c^2-6a=-1 \end{cases}

Given that a , b a, b and c c are real numbers, find the value of a b c \left|abc\right| .


The answer is 0.125.

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1 solution

Cheong Sik Feng
Sep 13, 2017

Relevant wiki: Completing the Square - Basic

By rearranging the equations,

{ 9 a 2 24 b + 17 = 0 16 b 2 + 4 c 7 = 0 4 c 2 6 a + 1 = 0 \begin{cases} 9a^2-24b+17=0 \\ 16b^2+4c-7=0 \\ 4c^2-6a+1=0 \end{cases}

9 a 2 24 b + 17 + 16 b 2 + 4 c 7 + 4 c 2 6 a + 1 = 0 9a^2-24b+17+16b^2+4c-7+4c^2-6a+1=0

9 a 2 6 a + 16 b 2 24 b + 4 c 2 + 4 c + 11 = 0 9a^2-6a+16b^2-24b+4c^2+4c+11=0

By completing squares,

( 9 a 2 6 a + 1 ) + ( 16 b 2 24 b + 9 ) + ( 4 c 2 + 4 c + 1 ) = 0 (9a^2-6a+1)+(16b^2-24b+9)+(4c^2+4c+1)=0

( 3 a 1 ) 2 + ( 4 b 3 ) 2 + ( 2 c + 1 ) 2 = 0 (3a-1)^2+(4b-3)^2+(2c+1)^2=0

\because A square number is always positive,

{ 3 a 1 = 0 a = 1 3 4 b 3 = 0 b = 3 4 2 c + 1 = 0 c = 1 2 \begin{cases} 3a-1=0 \Rightarrow a=\frac{1}{3} \\ 4b-3=0 \Rightarrow b=\frac{3}{4} \\ 2c+1=0 \Rightarrow c=-\frac{1}{2} \end{cases}

a b c = 1 8 = 0.125 \therefore |abc|=\frac{1}{8}=0.125

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