An algebra problem by Aira Thalca

Algebra Level 4

Three distinct integers a , b , c a, b, c satisfy the following three conditions :

  • a b c = 17955 abc = 17955
  • a , b , c a, b, c form an arithmetic sequence in that order, and
  • ( 3 a + b ) , ( 3 b + c ) , ( 3 c + a ) (3a + b), (3b + c), (3c + a) form a geometric sequence in that order.

What is the value of a + b + c a + b + c ?


The answer is -63.

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1 solution

Rishabh Jain
Dec 27, 2016

Let the integers be a , b = a + d , c = a + 2 d a,b=a+d,c=a+2d (since they form an AP). Since a b c = 17955 a ( a + d ) ( a + 2 d ) = 17955... ( 1 ) abc=17955\\ \implies a(a+d)(a+2d)=17955...(1)

( 3 a + b ) , ( 3 b + c ) , ( 3 c + a ) (3a + b), (3b + c), (3c + a) form a geometric sequence ( 3 b + c ) 2 = ( 3 a + b ) ( 3 c + a ) \implies (3b+c)^2=(3a+b)(3c+a) ( 4 a + 5 d ) 2 = ( 4 a + d ) ( 4 a + 6 d ) \implies (4a+5d)^2=(4a+d)(4a+6d)

19 d 2 + 12 a d = 0 a = 19 d 12 \implies 19d^2+12ad=0\implies a=\frac{-19d}{12} Substituting in ( 1 ) (1) we get:

19 d 12 7 d 12 5 d 12 = 17955 = 3 3 19 7 5 \frac{19d}{12}\cdot \frac{7d}{12}\cdot \frac{5d}{12}=17955=3^3\cdot 19\cdot 7\cdot 5

d = 36 a = 19 d 12 = 57 \implies d=36\implies a=\dfrac{-19d}{12}=-57

The sum of these terms is: a + b + c = 3 a + 3 d = 3 ( 36 + ( 57 ) ) = 63 a+b+c=3a+3d=3(36+(-57))=\boxed{\color{#69047E}{-63}}

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