Given four reals ( a , b , c , d ) . If ( b − c ) ( d − a ) ( a − b ) ( c − d ) = − 7 4 , then the value of ( a − b ) ( c − d ) ( a − c ) ( b − d ) = B A , where A , and B are coprime positive integer. If the value of A − B A + B = D C , where C , and D are coprime positive integer, find A + B + C + D .
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Given that ( b − c ) ( d − a ) ( a − b ) ( c − d ) = − 7 4 .
Let's do some manipulations.
( b − c ) ( d − a ) ( a − b ) ( c − d ) 7 ( a − b ) ( c − d ) 7 ( a c − a d − b c + b d ) 7 a c − 7 a d − 7 b c + 7 b d 1 1 a c − 1 1 a d − 1 1 b c + 1 1 b d 1 1 ( a c − a d − b c + b d ) 1 1 ( a − b ) ( c − d ) ( a − b ) ( c − d ) ( a − c ) ( b − d ) = − 7 4 = − 4 ( b − c ) ( d − a ) = − 4 ( b d − a b − c d + a c ) = − 4 b d + 4 a b + 4 c d − 4 a c ⇒ Adding both sides with 4 a c − 4 a d − 4 b c + 4 b d = 4 a b − 4 a d − 4 b c + 4 c d = 4 ( a b − a d − b c + c d ) = 4 ( a − c ) ( b − d ) = 4 1 1 = B A
We got A = 1 1 , B = 4 . Then, A − B A + B = 7 1 5 = D C .
We got C = 1 5 , D = 7 .
Hence, A + B + C + D = 3 7 .
A p p l y i n g c o m p o n e n d o − d i v i d e n d o , : − ( b − c ) ( d − a ) ( a − b ) ( c − d ) = − 7 4 , ⟹ b d + a c − a b − c d a c + b d − b c − a d = − 7 − 4 , ∴ b d + a c − a b − c d − a d − b c + a b + c d = 7 − 4 − 7 . . . . . . . U s i n g q p = s r , ∴ q p − q = s r − s . a n d b d + a c − a d − b c − a d − b c + a b + c d = − 4 − 1 1 . . . . . . . . . . U s i n g q p = s r , ∴ q p + q = s r + s . F a c t o r i z i n g ( a − d ) ( c − b ) ( a − c ) ( b − d ) = 4 1 1 . S o B A = 4 1 1 a n d s o A − B A + B = 1 1 − 4 1 1 + 4 = 7 1 5 = D C . . . . . . . . . U s i n g q p = s r , ∴ p − q p + q = r − s r + s . . ⟹ A + B + C + D = 1 1 + 4 + 1 5 + 7 = 3 7 .
α = ( b − c ) ( d − a ) ( a − b ) ( c − d ) ⇔ α 1 = ( a − b ) ( c − d ) ( b − c ) ( d − a ) β = ( a − b ) ( c − d ) ( a − c ) ( b − d ) ∴ α 1 + β = ( a − b ) ( c − d ) b d − a b − c d + a c + a b − a d − b c + c d = ( a − b ) ( c − d ) ( a − b ) ( c − d ) = 1 ⇒ β = 1 − α 1 = 1 + 4 7 = 4 1 1
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Let ( a − b ) = p ; ( c − d ) = q ; ( b − c ) = r ; ( d − a ) = s
a − b + b − c = a − c = p + r and c − d + d − a = c − a = q + s
So, a − c + c − a = p + q + r + s = 0
Now put these values in the given fraction r s p q = − 7 4
The fraction which is need to be evaluated becomes p q ( p + r ) ( q + r ) = p q p q + r ( p + q ) + r 2 = p q p q + r ( − r − s ) + r 2 . . . . . . . . . . . A s ( p + q + r + s ) = 0 = p q p q − r s = 1 − p q r s = 4 1 1