quadratic cocassi

Algebra Level 4

If A and B are the roots of x 2 + x + 1 = 0 x^2 + x +1 =0 and C and D are the roots of x 2 + 3 x + 1 = 0 x^2 +3x +1 =0 , then find the value of ( A C ) × ( B + D ) × ( A + D ) × ( B C ) (A-C) \times ( B+D) \times (A+D) \times (B-C) .


The answer is 8.

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3 solutions

Chew-Seong Cheong
Nov 29, 2014

Using Vieta's Formulas, we have:

A + B = 1 A B = 1 C + D = 3 C D = 1 A+B=-1\quad\quad AB = 1\quad \quad C+D = -3 \quad \quad CD=1

( A C ) ( B + D ) ( A + D ) ( B C ) = ( A B + A D B C C D ) ( A B A C + B D C D ) (A-C)(B+D)(A+D)(B-C) = (AB+AD-BC-CD)(AB-AC+BD-CD)

= ( 1 + A D B C 1 ) ( 1 A C + B D 1 ) = ( A D B C ) ( B D A C ) =(1+AD-BC-1)(1-AC+BD-1) = (AD-BC)(BD-AC)

= A B D 2 A 2 C D B 2 C D + A B C 2 = A B ( D 2 + C 2 ) C D ( A 2 + B 2 ) =ABD^2-A^2CD-B^2CD+ABC^2=AB(D^2+C^2)-CD(A^2+B^2)

= ( C 2 + D 2 ) ( A 2 + B 2 ) = [ ( C + D ) 2 2 C D ] [ ( A + B ) 2 2 A B ] =(C^2+D^2)-(A^2+B^2) = [(C+D)^2-2CD]-[(A+B)^2-2AB]

= 9 2 1 + 2 = 8 =9-2-1+2=\boxed{8}

Let P ( x ) = x 2 + x + 1 = ( x A ) ( x B ) P(x)=x^2+x+1=(x-A)(x-B) and Q ( x ) = x 2 + 3 x + 1 = ( x C ) ( x D ) Q(x)=x^2+3x+1=(x-C)(x-D) .

Now, rearrange the expression we want to find: ( C A ) ( C B ) ( D A ) ( D B ) (C-A)(C-B)(-D-A)(-D-B) , which is simply P ( C ) P ( D ) P(C)P(-D) :

( C 2 + C + 1 ) ( D 2 D + 1 ) (C^2+C+1)(D^2-D+1) .

Expanding we get:

1 + C 2 + D 2 + C D + ( C D ) 2 + C D 2 C 2 D C D 1+C^2+D^2+C-D+(CD)^2+CD^2-C^2D-CD

Simplifying it we get:

1 + ( C + D ) 2 3 C D + ( C D ) 2 ( C D ) ( 1 C D ) 1+(C+D)^2-3CD+(CD)^2-(C-D)(1-CD)

By Vieta's formula, we know that C + D = 3 C+D=-3 and C D = 1 CD=1 , so:

1 + ( 3 ) 2 3 ( 1 ) + ( 1 ) 2 ( C D ) ( 1 1 ) 1+(-3)^2-3(1)+(1)^2-(C-D)(1-1)

1 + 9 3 + 1 = 8 1+9-3+1=\boxed{8}

Nice solution. !

Sandeep Bhardwaj - 6 years, 6 months ago

Since C and D are roots of the 2nd equation, C^2 + 3C + 1 =0 and D^2 + 3D + 1 = 0. Using this we see that C^2 + C + 1 = -2C and D^2 - D + 1 = -4D. This becomes 8CD which equals 8 by Vieta's formula. I think this method saves the trouble of expanding once more.

Devin Ky - 5 years, 11 months ago
Jai Gupta
Nov 5, 2014

calin can use send me how to use latex in this question

For that wrap ur math part in \ ( \ ) Just don't put the space between \ and (

Aneesh Kundu - 6 years, 7 months ago

calvin not calin

math man - 6 years, 6 months ago

What is cocassi?

Joel Tan - 6 years, 6 months ago

Log in to reply

indian invention

Jai Gupta - 6 years, 6 months ago

Truthly Its my invention for algebra

Jai Gupta - 6 years, 6 months ago

Log in to reply

How did You invented it ????:P

Shubhendra Singh - 6 years, 5 months ago

There's a good latex guide here: https://brilliant.org/discussions/thread/beginner-latex-guide/?ref_id=498962

Marc Vince Casimiro - 6 years, 6 months ago

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