⎩ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎧ 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
If the above system of equations holds true for the positive numbers a , b , c and d , then find the value of the expression below.
1 1 a 2 + 1 0 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 + 2 1
P.S. Do you believe that the answer is a prime number?
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Same method! Took time to spot the trick, however! Awesome :)
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Tricky, right? That's why "The Solution is Unpredictable". Thanks
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⎩ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎧ 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 )
( 1 ) + 1 ⇒ a b c + a b + a c + b c + a + b + c + 1 = 8 → ( a + 1 ) ( b + 1 ) ( c + 1 ) = 8 . . . ( 5 ) .
( 2 ) + 1 ⇒ b c d + b c + b d + c d + b + c + d + 1 = 1 0 → ( b + 1 ) ( c + 1 ) ( d + 1 ) = 1 0 . . . ( 6 ) .
( 3 ) + 1 ⇒ c d a + c a + c d + d a + c + d + a + 1 = 1 0 → ( c + 1 ) ( d + 1 ) ( a + 1 ) = 1 0 . . . ( 7 ) .
( 4 ) + 1 ⇒ c d a + c a + c d + d a + c + d + a + 1 = 1 0 → ( d + 1 ) ( a + 1 ) ( b + 1 ) = 1 0 . . . ( 8 ) .
( 5 ) × ( 6 ) × ( 7 ) × ( 8 ) = ⇒ ( a + 1 ) ³ ( b + 1 ) ³ ( c + 1 ) ³ ( d + 1 ) ³ = 8 0 0 0 → ( a + 1 ) ( b + 1 ) ( c + 1 ) ( d + 1 ) = 2 0 . . . ( 9 ) .
( 9 ) : ( 1 ) gives d = 2 3 .
( 9 ) : ( 2 ) gives a = 1 .
( 9 ) : ( 3 ) gives b = 1 .
( 9 ) : ( 4 ) gives c = 1 .
Then, we have 1 1 a 2 + 1 0 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 + 2 1
= 1 1 + 1 0 + 9 + 1 8 + 7 + 6 + 2 1 5 + 6 + 2 9 + 2 + 2 3 + 2 1
Hence, our final answer 8 3 .