"Unpredictable Solution" Problem (Part 1)

Algebra Level 4

{ a b c + a b + a c + b c + a + b + c = 7 b c d + b c + b d + c d + b + c + d = 9 c d a + c a + c d + d a + c + d + a = 9 d a b + d a + d b + a b + d + a + b = 9 \begin{cases} abc + ab + ac + bc + a+ b + c = 7 \\ bcd + bc + bd + cd + b + c + d = 9 \\ cda + ca + cd + da + c + d + a = 9 \\ dab + da + db + ab + d + a + b = 9 \end{cases}

If the above system of equations holds true for the positive numbers a , b , c a, b, c and d d , then find the value of the expression below.

11 a 2 + 10 b 2 + 9 c 2 + 8 d 2 + 7 a b + 6 b c + 5 c d + 4 d a + 3 d b + 2 a c + a b c d + 1 2 \begin{aligned} 11a^2 + 10b^2 + 9c^2 + 8d^2 + 7ab + 6bc + 5cd + 4da + 3db + 2ac + abcd + \frac{1}{2} \end{aligned}

P.S. Do you believe that the answer is a prime number?


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The answer is 83.

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1 solution

Fidel Simanjuntak
Jan 24, 2017

{ a b c + a b + a c + b c + a + b + c = 7 . . . ( 1 ) b c d + b c + b d + c d + b + c + d = 9 . . . ( 2 ) c d a + c a + c d + d a + c + d + a = 9 . . . ( 3 ) c d a + c a + c d + d a + c + d + a = 9 . . . ( 4 ) \begin{cases} abc + ab + ac + bc + a+ b + c = 7 \space ...(1) \\ bcd + bc + bd + cd + b + c + d = 9 \space ...(2) \\ cda + ca + cd + da + c + d + a = 9 \space ...(3) \\ cda + ca + cd + da + c + d + a = 9\space ...(4) \end{cases}

( 1 ) + 1 a b c + a b + a c + b c + a + b + c + 1 = 8 ( a + 1 ) ( b + 1 ) ( c + 1 ) = 8 . . . ( 5 ) (1) + 1 \Rightarrow \space abc + ab + ac + bc + a+ b + c + 1 = 8 \rightarrow (a+1)(b+1)(c+1) = 8 \space ...(5) .

( 2 ) + 1 b c d + b c + b d + c d + b + c + d + 1 = 10 ( b + 1 ) ( c + 1 ) ( d + 1 ) = 10 . . . ( 6 ) (2) + 1 \Rightarrow \space bcd + bc + bd + cd + b + c + d + 1= 10 \rightarrow (b+1)(c+1)(d+1) = 10 \space ...(6) .

( 3 ) + 1 c d a + c a + c d + d a + c + d + a + 1 = 10 ( c + 1 ) ( d + 1 ) ( a + 1 ) = 10 . . . ( 7 ) (3) + 1 \Rightarrow \space cda + ca + cd + da + c + d + a +1 = 10 \rightarrow (c+1)(d+1)(a+1) = 10 \space ...(7) .

( 4 ) + 1 c d a + c a + c d + d a + c + d + a + 1 = 10 ( d + 1 ) ( a + 1 ) ( b + 1 ) = 10 . . . ( 8 ) (4) + 1 \Rightarrow \space cda + ca + cd + da + c + d + a +1 = 10 \rightarrow (d+1)(a+1)(b+1) = 10 \space ...(8) .

( 5 ) × ( 6 ) × ( 7 ) × ( 8 ) = ( a + 1 ) ³ ( b + 1 ) ³ ( c + 1 ) ³ ( d + 1 ) ³ = 8000 ( a + 1 ) ( b + 1 ) ( c + 1 ) ( d + 1 ) = 20 . . . ( 9 ) (5) \times (6) \times (7) \times (8) = \Rightarrow \space (a+1)³(b+1)³(c+1)³(d+1)³ = 8000 \rightarrow \space (a+1)(b+1)(c+1)(d+1) = 20 \space ...(9) .

( 9 ) : ( 1 ) (9) : (1) gives d = 3 2 d = \frac{3}{2} .

( 9 ) : ( 2 ) (9) : (2) gives a = 1 a =1 .

( 9 ) : ( 3 ) (9) : (3) gives b = 1 b =1 .

( 9 ) : ( 4 ) (9) : (4) gives c = 1 c = 1 .

Then, we have 11 a 2 + 10 b 2 + 9 c 2 + 8 d 2 + 7 a b + 6 b c + 5 c d + 4 d a + 3 d b + 2 a c + a b c d + 1 2 11a^2 + 10b^2 + 9c^2 + 8d^2 + 7ab + 6bc + 5cd + 4da + 3db + 2ac + abcd + \frac{1}{2}

= 11 + 10 + 9 + 18 + 7 + 6 + 15 2 + 6 + 9 2 + 2 + 3 2 + 1 2 = 11 + 10 + 9 + 18 + 7 + 6 + \frac{15}{2} + 6 + \frac{9}{2} + 2 + \frac{3}{2} + \frac{1}{2}

Hence, our final answer 83 \boxed{83} .

Same method! Took time to spot the trick, however! Awesome :)

Yatin Khanna - 4 years, 4 months ago

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Tricky, right? That's why "The Solution is Unpredictable". Thanks

Fidel Simanjuntak - 4 years, 4 months ago

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