⎩ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎧ a 2 − b 2 c 2 + d 2 ( a c ) 2 − ( b d ) 2 ( a d ) 2 − ( b c ) 2 = = = = 5 7 4 3 4 1 ?
Considering the constraints above, what is the value of the last equation?
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Let matrix A = [ a c b d ] and matrix B = [ c d − a b ] .
Then multiplying these matrices, we will obtain: A × B = [ a c b d ] ⋅ [ c d − a b ] = [ a c + b d c 2 + d 2 b 2 − a 2 b d − a c ]
Calculating the Determinants of these matrices, we know that the det ( A B ) = det ( A ) ⋅ det ( B ) .
Hence, ( a d − b c ) ( a d + b c ) = ( a c + b d ) ( b d − a c ) − ( b 2 − a 2 ) ( c 2 + d 2 ) .
( a d ) 2 − ( b c ) 2 = [ ( b d ) 2 − ( a c ) 2 ] + ( a 2 − b 2 ) ( c 2 + d 2 ) = − 3 4 1 + 5 × 7 4 = 3 7 0 − 3 4 1 = 2 9 .
I think it's more natural to see the factorization ( a 2 − b 2 ) ( c 2 + d 2 ) = ( a c ) 2 − ( b d ) 2 + ( a d ) 2 − ( b c ) 2 directly.
To me, representing it in matrix form obscures what is going on.
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Well, in a way. I just wanted a problem with matrix determinant and just came up with this.
Similar to Lagrange's identity!
Using matrices looks much more complicated but, on the other hand, gives a more generalizing approach to problem. That's my take.
Just multiplied the first 2 equations and that's it.
(+-3,+-2,+-7,+-5)
Yes, we can factorize those equations.
a ² − b ² = 5
( a + b ) ( a − b ) = 5 × 1
a + b = 5 ; a − b = 1
Solving, we get a = 3 and b = 2 .
( a c ) ² − ( b d ) ² = 3 4 1
( 3 c ) ² − ( 2 d ) ² = 3 4 1
( 3 c + 2 b ) ( 3 c − 2 b ) = 3 4 1
Since 3 c + 2 b > 3 c − 2 b , then we have
3 c + 2 b = 3 4 1 or 3 1 ; and 3 c − 2 b = 1 or 1 1
First condition. If 3 c + 2 b = 3 4 1 and 3 c − 2 b = 1 ,
then c ² + d ² = 7 4 .
Second condition; 3 c + 2 b = 3 1 and 3 c − 2 b = 1 1 , solving we get c = 7 and d = 5 .
Then, the second equation is completed.
Now,
( a d ) ² − ( b c ) ² = ( 3 × 5 ) ² − ( 2 × 7 ) ²
= ( 1 5 + 1 4 ) ( 1 5 − 1 4 ) = 2 9 × 1
= 2 9
Technically you are not given that a,b,c,d are integers. But if you are told the answer is unique this is valid.
There are only 3 equations given so you have 1 degree of freedom for the values of a 2 , b 2 , c 2 , d 2 . So you can also solve by setting b = 0 :
a 2 = 5
5 c 2 = 3 4 1 = > c 2 = 6 8 . 2
c 2 + d 2 = 7 4 = > d 2 = 5 . 8
( a d ) 2 − ( b c ) 2 = 5 d 2 = 2 9
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Here is a simple solution, without matrices and determinants:
Let's multiply the first two equations and take away the third from their product:
( a 2 − b 2 ) ( c 2 + d 2 ) − ( ( a c ) 2 − ( b d ) 2 ) = 5 × 7 4 − 3 4 1
( a c ) 2 − ( b c ) 2 + ( a d ) 2 − ( b d ) 2 − ( a c ) 2 + ( b d ) 2 = 3 7 0 − 3 4 1
After simplifying, we get our fourth equation with the missing value:
( a d ) 2 − ( b c ) 2 = 2 9