1729

Algebra Level 5

a b c + a b + b c + c a + a + b + c = 1 b c d + b c + c d + d b + b + c + d = 7 c d a + c d + d a + a c + c + d + a = 2 d a b + d a + a b + b d + d + a + b = 9 \begin{aligned} abc+ab+bc+ca+a+b+c&=&1\\ bcd+bc+cd+db+b+c+d&=&7 \\ cda+cd+da+ac+c+d+a&=&2\\ dab+da+ab+bd+d+a+b&=&9 \end{aligned}

There exist 4 non-real numers a , b , c a,b,c and d d satisfying the system of equations above, find the value of ( ( a b ) ( a d ) ( c b ) ( c d ) ) 3 ((a-b)(a-d)(c-b)(c-d))^3 .


please try its sequel here


The answer is 20580.0.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Otto Bretscher
Dec 1, 2015

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 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 = 10 ABC=2, BCD=8, ACD=3, ABD=10 Multiplying these together, we find that ( A B C D ) 3 = 2 8 3 10 = 480 (ABCD)^3=2*8*3*10=480 .

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 (A-B)CD=3-8=-5, (A-D)BC=2-8=-6, (C-B)AD=-7, (C-D)AB=-8 Multiplying these together, we find that ( A B ) ( A D ) ( C B ) ( C D ) ( A B C D ) 2 = 5 6 7 8 = 1680 (A-B)(A-D)(C-B)(C-D)(ABCD)^2=5*6*7*8=1680

Finally, ( ( A B ) ( A D ) ( C B ) ( C D ) ) 3 = 168 0 3 48 0 2 = 20580 ((A-B)(A-D)(C-B)(C-D))^3=\frac{1680^3}{480^2}=\boxed{20580}

Moderator note:

That's a very nice way of calculating ( A B ) (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 D A B C = 480 3 2 D = \frac{ ABCD}{ABC} = \frac{ \sqrt[3]{480} }{2} , etc, and that these solutions satisfy the original system of equations.

Aareyan Manzoor
Nov 29, 2015

add one to each eqn to get: ( a + 1 ) ( b + 1 ) ( c + 1 ) = 2 (a+1)(b+1)(c+1)=2 ( b + 1 ) ( c + 1 ) ( d + 1 ) = 8 (b+1)(c+1)(d+1)=8 ( c + 1 ) ( d + 1 ) ( a + 1 ) = 3 (c+1)(d+1)(a+1)=3 ( d + 1 ) ( a + 1 ) ( b + 1 ) = 10 (d+1)(a+1)(b+1)=10 multiply all and square both sides to get ( ( a + 1 ) ( b + 1 ) ( c + 1 ) ( d + 1 ) ) 6 = ( 2 8 3 10 ) 2 ((a+1)(b+1)(c+1)(d+1))^6=(2*8*3*10)^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-d)(bc+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 (a-b)(cd+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-b)(ad+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 (c-d)(ab+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 = 168 0 3 ((a-b)(a-d)(c-b)(c-d))^3((a+1)(b+1)(c+1)(d+1))^6=1680^3 substitute value : ( ( a b ) ( a d ) ( c b ) ( c d ) ) 3 = 168 0 3 ( 2 8 3 10 ) 2 = 20580 ((a-b)(a-d)(c-b)(c-d))^3=\dfrac{1680^3}{(2*8*3*10)^2}=\boxed{20580}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...