A algebra problem by Norwyn Kah

Algebra Level 4

Let real numbers a a , b b and c c satisfy a + b + c = a 2 + b 2 + c 2 = 2 a+b+c=a^2+b^2+c^2=2 .

Find ( 1 a ) 2 b c + ( 1 b ) 2 a c + ( 1 c ) 2 a b \dfrac{(1-a)^2}{bc}+\dfrac{(1-b)^2}{ac}+\dfrac{(1-c)^2}{ab} .


The answer is 3.

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1 solution

Norwyn Kah
May 30, 2016

squaring a + b + c = 2 a+b+c=2 will become a 2 + b 2 + c 2 + 2 a b + 2 a c + 2 b c = 4 a^2+b^2+c^2+2ab+2ac+2bc=4

comparing this equation with a 2 + b 2 + c 2 = 2 a^2+b^2+c^2=2 will give us the equation a b + b c + a c = 1 ab+bc+ac=1

expanding the equation we need to find will become a 3 + b 3 + c 3 2 a 2 2 b 2 2 c 2 + a + b + c a b c \frac{a^3+b^3+c^3-2a^2-2b^2-2c^2+a+b+c}{abc}

a 3 + b 3 + c 3 2 ( 2 ) + 2 a b c \Rightarrow\frac{a^3+b^3+c^3-2(2)+2}{abc}

a 3 + b 3 + c 3 2 a b c \Rightarrow\frac{a^3+b^3+c^3-2}{abc}

using the identity a 3 + b 3 + c 3 = ( a + b + c ) 3 3 ( a + b ) ( b + c ) ( c + a ) a^3+b^3+c^3=(a+b+c)^3-3(a+b)(b+c)(c+a)

a 3 + b 3 + c 3 = 8 3 ( 2 c ) ( 2 b ) ( 2 a ) \Rightarrow a^3+b^3+c^3=8-3(2-c)(2-b)(2-a)

a 3 + b 3 + c 3 = 8 3 ( 8 4 ( a + b + c ) + 2 ( a b + b c + a c ) a b c ) \Rightarrow a^3+b^3+c^3=8-3(8-4(a+b+c)+2(ab+bc+ac)-abc)

a 3 + b 3 + c 3 = 8 3 ( 8 4 ( 2 ) + 2 ( 1 ) a b c ) \Rightarrow a^3+b^3+c^3=8-3(8-4(2)+2(1)-abc)

a 3 + b 3 + c 3 = 8 3 ( 2 a b c ) \Rightarrow a^3+b^3+c^3=8-3(2-abc)

a 3 + b 3 + c 3 = 2 + 3 a b c \Rightarrow a^3+b^3+c^3=2+3abc

substituting the value of a 3 + b 3 + c 3 a^3+b^3+c^3 in a 3 + b 3 + c 3 2 a b c \frac{a^3+b^3+c^3-2}{abc}

2 + 3 a b c 2 a b c \Rightarrow\frac{2+3abc-2}{abc}

3 a b c a b c \Rightarrow\frac{3abc}{abc}

3 \Rightarrow\boxed{3}

Same approach (+1)

Grant Bulaong - 5 years ago

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