a,b and c real numbers

Algebra Level pending

{ a ( b + c ) = 152 b ( a + c ) = 162 c ( a + b ) = 170 \begin{cases} a( b+c)=152 \\ b(a+c)= 162 \\ c(a+b)=170 \end{cases}

If a a , b b , and c c are positive real numbers satisfying the system of equations, find a b c abc .


The answer is 720.

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3 solutions

Steven Chase
Sep 20, 2016

Distributing gives:

a b + a c = 152 ab + ac = 152
a b + b c = 162 ab + bc = 162
a c + b c = 170 ac + bc = 170

Subtracting the first from the second gives:

b c a c = 10 bc - ac = 10
a c + b c = 170 ac + bc = 170

Solving for b c bc , and subsequently a b ab and a c ac :
b c = 90 , a b = 72 , a c = 80 bc = 90,\, ab = 72,\,ac=80

Then, a b c = a 2 b 2 c 2 = a b a c b c = 90 72 80 = 720 abc = \sqrt{a^{2}b^{2}c^{2}} = \sqrt{ab*ac*bc} = \sqrt{90 * 72 * 80} = 720

Chew-Seong Cheong
Sep 20, 2016

{ a ( b + c ) = a b + c a = 152 . . . ( 1 ) b ( a + c ) = a b + b c = 162 . . . ( 2 ) c ( a + b ) = c a + b c = 170 . . . ( 3 ) \begin{cases} a(b+c) = ab+ca = 152 &...(1) \\ b(a+c) = ab+bc = 162 & ...(2) \\ c(a+b) = ca+bc = 170 &...(3) \end{cases}

( 1 ) + ( 2 ) ( 3 ) : 2 a b = 144 a b = 72 . . . ( 4 ) \begin{aligned} (1)+(2)-(3): \quad 2ab & = 144 \\ ab & = 72 & ...(4) \end{aligned}

( 1 ) : c a = 152 72 = 80 . . . ( 5 ) ( 2 ) : b c = 162 72 = 90 . . . ( 6 ) \begin{aligned} (1): \quad ca & = 152-72 = 80 & ...(5) \\ (2): \quad bc & = 162-72 = 90 & ...(6) \end{aligned}

( 4 ) ( 5 ) ( 6 ) : ( a b c ) 2 = 72 80 90 a b c = 720 \begin{aligned} (4)(5)(6): \quad (abc)^2 & = 72\cdot 80 \cdot 90 \\ abc & = \boxed{720} \end{aligned}

William Rockman
Aug 15, 2018

ab+ac=152. A ba+bc=162. B ca+cb=170. C

Therefore, a=152/b+c, b=162/a+c, c=170/a+b

Taking A and substituting b: a(162/a+c) + ac = 152, 162a/a+c + ac = 152, 162a + ac(a+c) = 152a + 152c, 162a + ca^2 + ac^2 = 152a + 152c, ac^2 + ca^2 + 10a - 152c = 0

Taking C and substituting b: ca + c(162/a+c) = 170, ca + 162c/a+c = 170, ca(a+c) + 162c = 170a + 170c, ca^2 + ac^2 + 162c = 170a + 170c, ac^2 + ca^2 - 8c - 170 = 0

Clearly: 10a - 152c = -8c - 170a, 180a = 144c, 15a = 12c, 5a = 4c, a = 4c/5

Taking b=162/a+c: b = 162/(4c/5 + c), b = 162/(9c/5), 9c/5 = 162/b, 9cb = 810, bc = 90, b = 90/c

Taking A: ab + ac = 152, (4c/5)(90/c) + (4c/5)c = 152, 72 + 4c^2/5 = 760, 4c^2 = 400, c^2 = 100, c = 10. (Note: question says positive integers so c can't be -10) And so using c=10, b=9, and a=8.

abc= 720 😃😃😃

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