a b c + a b + b c + c a + a + b + c b c d + b c + c d + d b + b + c + d c d a + c d + d a + a c + c + d + a d a b + d a + a b + b d + d + a + b = = = = 1 7 2 9
There exist 4 non-real numers a , b , c and d satisfying the system of equations above, find the value of ( ( a − b ) ( a − d ) ( c − b ) ( c − d ) ) 3 .
please try its sequel here
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That's a very nice way of calculating ( A − B ) , without finding out the explicit value.
For completeness, one should ensure that there are indeed solutions to the system of equations. E.g. We have D = A B C A B C D = 2 3 4 8 0 , etc, and that these solutions satisfy the original system of equations.
add one to each eqn to get: ( a + 1 ) ( b + 1 ) ( c + 1 ) = 2 ( b + 1 ) ( c + 1 ) ( d + 1 ) = 8 ( c + 1 ) ( d + 1 ) ( a + 1 ) = 3 ( d + 1 ) ( a + 1 ) ( b + 1 ) = 1 0 multiply all and square both sides to get ( ( a + 1 ) ( b + 1 ) ( c + 1 ) ( d + 1 ) ) 6 = ( 2 ∗ 8 ∗ 3 ∗ 1 0 ) 2 now subtract the original equation given as pairs: ( a − d ) ( b c + c + b + 1 ) = ( d − a ) ( b + 1 ) ( c + 1 ) = 1 − 9 = − 8 ( a − b ) ( c d + c + d + 1 ) = ( b − a ) ( c + 1 ) ( d + 1 ) = 1 − 7 = − 6 ( c − b ) ( a d + a + d + 1 ) = ( b − c ) ( a + 1 ) ( d + 1 ) = 2 − 7 = − 5 ( c − d ) ( a b + a + b + 1 ) = ( d − c ) ( a + 1 ) ( b + 1 ) = 2 − 9 = − 7 multiply these and cube both sides to get ( ( a − b ) ( a − d ) ( c − b ) ( c − d ) ) 3 ( ( a + 1 ) ( b + 1 ) ( c + 1 ) ( d + 1 ) ) 6 = 1 6 8 0 3 substitute value : ( ( a − b ) ( a − d ) ( c − b ) ( c − d ) ) 3 = ( 2 ∗ 8 ∗ 3 ∗ 1 0 ) 2 1 6 8 0 3 = 2 0 5 8 0
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My solution is similar to Aareyan's, but becoming a bit more transparent through the use of substitutions.
Let A = a + 1 , B = b + 1 , C = c + 1 , D = d + 1 We are told that A B C = 2 , B C D = 8 , A C D = 3 , A B D = 1 0 Multiplying these together, we find that ( A B C D ) 3 = 2 ∗ 8 ∗ 3 ∗ 1 0 = 4 8 0 .
Now ( A − B ) C D = 3 − 8 = − 5 , ( A − D ) B C = 2 − 8 = − 6 , ( C − B ) A D = − 7 , ( C − D ) A B = − 8 Multiplying these together, we find that ( A − B ) ( A − D ) ( C − B ) ( C − D ) ( A B C D ) 2 = 5 ∗ 6 ∗ 7 ∗ 8 = 1 6 8 0
Finally, ( ( A − B ) ( A − D ) ( C − B ) ( C − D ) ) 3 = 4 8 0 2 1 6 8 0 3 = 2 0 5 8 0