Switching some Variables

Algebra Level 5

Given four reals ( a , b , c , d ) (a,b,c,d) . If ( a b ) ( c d ) ( b c ) ( d a ) = 4 7 \dfrac{(a-b)(c-d)}{(b-c)(d-a)} = -\dfrac{4}{7} , then the value of ( a c ) ( b d ) ( a b ) ( c d ) = A B \dfrac{(a-c)(b-d)}{(a-b)(c-d)} = \dfrac{A}{B} , where A , A, and B B are coprime positive integer. If the value of A + B A B = C D \dfrac{A+B}{A-B} = \dfrac{C}{D} , where C , C, and D D are coprime positive integer, find A + B + C + D A+B+C+D .


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The answer is 37.

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4 solutions

Kushal Bose
Apr 30, 2017

Let ( a b ) = p ; ( c d ) = q ; ( b c ) = r ; ( d a ) = s (a-b)=p\,\,; (c-d)=q\,\,; (b-c)=r\,\, ; (d-a)=s

a b + b c = a c = p + r a-b+b-c=a-c=p+r and c d + d a = c a = q + s c-d+d-a=c-a=q+s

So, a c + c a = p + q + r + s = 0 a-c+c-a=p+q+r+s=0

Now put these values in the given fraction p q r s = 4 7 \dfrac{pq}{rs}=-\dfrac{4}{7}

The fraction which is need to be evaluated becomes ( p + r ) ( q + r ) p q = p q + r ( p + q ) + r 2 p q = p q + r ( r s ) + r 2 p q . . . . . . . . . . . A s ( p + q + r + s ) = 0 = p q r s p q = 1 r s p q = 11 4 \dfrac{(p+r)(q+r)}{pq} \\ = \dfrac{pq+r(p+q) + r^2}{pq} \\= \dfrac{pq+r(-r-s) +r^2}{pq}...........As\,\, (p+q+r+s)=0 \\ =\dfrac{pq -rs}{pq} \\ =1-\dfrac{rs}{pq} =\dfrac{11}{4}

Fidel Simanjuntak
Apr 30, 2017

Given that ( a b ) ( c d ) ( b c ) ( d a ) = 4 7 \dfrac{(a-b)(c-d)}{(b-c)(d-a)} = -\dfrac{4}{7} .

Let's do some manipulations.

( a b ) ( c d ) ( b c ) ( d a ) = 4 7 7 ( a b ) ( c d ) = 4 ( b c ) ( d a ) 7 ( a c a d b c + b d ) = 4 ( b d a b c d + a c ) 7 a c 7 a d 7 b c + 7 b d = 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 11 a c 11 a d 11 b c + 11 b d = 4 a b 4 a d 4 b c + 4 c d 11 ( a c a d b c + b d ) = 4 ( a b a d b c + c d ) 11 ( a b ) ( c d ) = 4 ( a c ) ( b d ) ( a c ) ( b d ) ( a b ) ( c d ) = 11 4 = A B \begin{aligned} \dfrac{(a-b)(c-d)}{(b-c)(d-a)} & = -\dfrac{4}{7} \\ 7(a-b)(c-d) & = -4(b-c)(d-a) \\ 7(ac - ad - bc + bd ) & = -4(bd - ab - cd + ac) \\ 7ac - 7ad - 7bc + 7 bd & = -4bd + 4ab + 4cd - 4ac \quad \color{#3D99F6} \Rightarrow \text{Adding both sides with } \space 4ac - 4ad -4bc + 4bd \\ \color{#333333} 11ac - 11ad - 11bc + 11bd & = 4ab - 4ad -4bc + 4cd \\ 11(ac - ad - bc + bd) & = 4( ab - ad - bc + cd) \\ 11(a-b)(c-d) & = 4(a-c)(b-d) \\ \dfrac{(a-c)(b-d)}{(a-b)(c-d)} & = \dfrac{11}{4} = \dfrac{A}{B} \end{aligned}

We got A = 11 , B = 4 A= 11, \space B = 4 . Then, A + B A B = 15 7 = C D \dfrac{A+B}{A-B} = \dfrac{15}{7} = \dfrac{C}{D} .

We got C = 15 , D = 7 C = 15, \space D = 7 .

Hence, A + B + C + D = 37 A+B+C+D = 37 .

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 , : ( a b ) ( c d ) ( b c ) ( d a ) = 4 7 , a c + b d b c a d b d + a c a b c d = 4 7 , a d b c + a b + c d b d + a c a b c d = 4 7 7 . . . . . . . U s i n g p q = r s , p q q = r s s . a n d a d b c + a b + c d b d + a c a d b c = 11 4 . . . . . . . . . . U s i n g p q = r s , p + q q = r + s s . F a c t o r i z i n g ( a c ) ( b d ) ( a d ) ( c b ) = 11 4 . S o A B = 11 4 a n d s o A + B A B = 11 + 4 11 4 = 15 7 = C D . . . . . . . . . U s i n g p q = r s , p + q p q = r + s r s . . A + B + C + D = 11 + 4 + 15 + 7 = 37. Applying~componendo-dividendo,:-\\ \dfrac{(a-b)(c-d)}{(b-c)(d-a)} = -\dfrac{4}{7},~~\implies~~\dfrac{ac+bd-bc-ad}{bd+ac-ab-cd}=-\dfrac {-4} 7,\\ \therefore~\dfrac{-ad-bc+ab+cd}{bd+ac-ab-cd}=\dfrac{-4-7} 7.......Using~~\dfrac p q=\dfrac r s, ~~\therefore~\dfrac {p-q} q=\dfrac{r-s} s.\\ and ~~ \dfrac{-ad-bc+ab+cd}{bd+ac-ad-bc}=\dfrac{-11} {-4}..........Using~~\dfrac p q=\dfrac r s, ~~\therefore~\dfrac {p+q} q=\dfrac{r+s} s.\\ Factorizing~~\dfrac{(a-c)(b-d)}{(a-d)(c-b)}=\dfrac{11} 4.\\ So~~\dfrac A B=\dfrac{11} 4~~\\ and~~so~~\dfrac{A+B}{A-B}=\dfrac{11+4}{11-4}=\dfrac {15} 7=\dfrac C D.........Using~~ \dfrac p q=\dfrac r s,~~\therefore~\dfrac{p+q}{p-q}=\dfrac{r+s}{r-s}..\\ \implies~~A+B+C+D=11+4+15+7=\Large~~\color{#D61F06}{37}.

Aditya Dhawan
May 6, 2017

α = ( a b ) ( c d ) ( b c ) ( d a ) 1 α = ( b c ) ( d a ) ( a b ) ( c d ) β = ( a c ) ( b d ) ( a b ) ( c d ) 1 α + β = b d a b c d + a c + a b a d b c + c d ( a b ) ( c d ) = ( a b ) ( c d ) ( a b ) ( c d ) = 1 β = 1 1 α = 1 + 7 4 = 11 4 \alpha =\quad \frac { (a-b)(c-d) }{ (b-c)(d-a) } \Leftrightarrow \quad \frac { 1 }{ \alpha } =\quad \frac { (b-c)(d-a) }{ (a-b)(c-d) } \\ \beta =\quad \frac { (a-c)(b-d) }{ (a-b)(c-d) } \\ \therefore \quad \frac { 1 }{ \alpha } +\beta \quad =\frac { bd-ab-cd+ac+ab-ad-bc+cd }{ (a-b)(c-d) } =\frac { (a-b)(c-d) }{ (a-b)(c-d) } \quad =1\\ \Rightarrow \quad \beta =\quad 1-\frac { 1 }{ \alpha } =\quad 1+\frac { 7 }{ 4 } =\boxed { \frac { 11 }{ 4 } }

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