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Algebra Difficulty 4.3 AIME Find the answer United States

Integers aa, bb, and cc satisfy ab+c=100ab + c = 100, bc+a=87bc + a = 87, and ca+b=60ca + b = 60. What is ab+bc+caab + bc + ca?

Pick one

Solution

Answer (D): Notice that the difference between 100100 and 8787 is 1313, a prime number. This fact will help to simplify the problem. Subtract the second equation from the first to get
13=(ab+c)(bc+a)=abbca+c=b(ac)(ac)=(b1)(ac). \begin{aligned} 13 &= (ab + c) - (bc + a) \\ &= ab - bc - a + c \\ &= b(a - c) - (a - c) \\ &= (b - 1)(a - c). \end{aligned}
Thus b1=±1b - 1 = \pm 1 or b1=±13b - 1 = \pm 13.

* If b1=1b - 1 = -1, then b=0b = 0, implying c=100c = 100, a=87a = 87, and ca=60ca = 60, which is impossible.
* If b1=1b - 1 = 1, then b=2b = 2 and ac=13a - c = 13, implying ca=58=229ca = 58 = 2 \cdot 29, which cannot be true if ac=13a - c = 13.
* If b1=13b - 1 = 13, then b=14b = 14 and ac=1a - c = 1, implying ca=46=223ca = 46 = 2 \cdot 23, which cannot be true if ac=1a - c = 1.
* If b1=13b - 1 = -13, then b=12b = -12 and ac=1a - c = -1, implying ca=72ca = 72, which is satisfied when a=9a = -9 and c=8c = -8. In fact, a=9a = -9, b=12b = -12, and c=8c = -8 satisfies all three equations.

The requested value is ab+bc+ca=(9)(12)+(12)(8)+(8)(9)=108+96+72=276ab + bc + ca = (-9)(-12) + (-12)(-8) + (-8)(-9) = 108 + 96 + 72 = 276.

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