⎩ ⎪ ⎨ ⎪ ⎧ a ( b + c ) = 1 5 2 b ( a + c ) = 1 6 2 c ( a + b ) = 1 7 0
If a , b , and c are positive real numbers satisfying the system of equations, find a b c .
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⎩ ⎪ ⎨ ⎪ ⎧ a ( b + c ) = a b + c a = 1 5 2 b ( a + c ) = a b + b c = 1 6 2 c ( a + b ) = c a + b c = 1 7 0 . . . ( 1 ) . . . ( 2 ) . . . ( 3 )
( 1 ) + ( 2 ) − ( 3 ) : 2 a b a b = 1 4 4 = 7 2 . . . ( 4 )
( 1 ) : c a ( 2 ) : b c = 1 5 2 − 7 2 = 8 0 = 1 6 2 − 7 2 = 9 0 . . . ( 5 ) . . . ( 6 )
( 4 ) ( 5 ) ( 6 ) : ( a b c ) 2 a b c = 7 2 ⋅ 8 0 ⋅ 9 0 = 7 2 0
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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Distributing gives:
a b + a c = 1 5 2
a b + b c = 1 6 2
a c + b c = 1 7 0
Subtracting the first from the second gives:
b c − a c = 1 0
a c + b c = 1 7 0
Solving for b c , and subsequently a b and a c :
b c = 9 0 , a b = 7 2 , a c = 8 0
Then, a b c = a 2 b 2 c 2 = a b ∗ a c ∗ b c = 9 0 ∗ 7 2 ∗ 8 0 = 7 2 0