If A and B are the roots of x 2 + x + 1 = 0 and C and D are the roots of x 2 + 3 x + 1 = 0 , then find the value of ( A − C ) × ( B + D ) × ( A + D ) × ( B − C ) .
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Let P ( x ) = x 2 + x + 1 = ( x − A ) ( x − B ) and Q ( x ) = x 2 + 3 x + 1 = ( x − C ) ( x − D ) .
Now, rearrange the expression we want to find: ( C − A ) ( C − B ) ( − D − A ) ( − D − B ) , which is simply P ( C ) P ( − D ) :
( 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
Simplifying it we get:
1 + ( C + D ) 2 − 3 C D + ( C D ) 2 − ( C − D ) ( 1 − C D )
By Vieta's formula, we know that C + D = − 3 and C D = 1 , so:
1 + ( − 3 ) 2 − 3 ( 1 ) + ( 1 ) 2 − ( C − D ) ( 1 − 1 )
1 + 9 − 3 + 1 = 8
Nice solution. !
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.
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For that wrap ur math part in \ ( \ ) Just don't put the space between \ and (
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There's a good latex guide here: https://brilliant.org/discussions/thread/beginner-latex-guide/?ref_id=498962
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Using Vieta's Formulas, we have:
A + B = − 1 A B = 1 C + D = − 3 C D = 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 )
= ( 1 + A D − B C − 1 ) ( 1 − A C + B D − 1 ) = ( A D − B C ) ( B D − A C )
= 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 )
= ( C 2 + D 2 ) − ( A 2 + B 2 ) = [ ( C + D ) 2 − 2 C D ] − [ ( A + B ) 2 − 2 A B ]
= 9 − 2 − 1 + 2 = 8