Can you factorize these squares?

Algebra Level 2

{ a 2 b 2 = 5 c 2 + d 2 = 74 ( a c ) 2 ( b d ) 2 = 341 ( a d ) 2 ( b c ) 2 = ? {\begin{cases} a^2 - b^2 &=& 5 \\ c^2 + d^2 &=& 74 \\ (ac)^2 - (bd)^2 &=& 341 \\ (ad)^2 - (bc)^2 &=& ? \end{cases} }

Considering the constraints above, what is the value of the last equation?


The answer is 29.

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3 solutions

Zee Ell
Dec 5, 2016

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 × 74 341 (a^2 - b^2)(c^2 + d^2) - ( (ac)^2 - (bd)^2 ) = 5 × 74 - 341

( a c ) 2 ( b c ) 2 + ( a d ) 2 ( b d ) 2 ( a c ) 2 + ( b d ) 2 = 370 341 (ac)^2 - (bc)^2 + (ad)^2 - (bd)^2 - (ac)^2 + (bd)^2 = 370 - 341

After simplifying, we get our fourth equation with the missing value:

( a d ) 2 ( b c ) 2 = 29 (ad)^2 - (bc)^2 = \boxed {29}

Let matrix A = [ a b c d ] A = \begin{bmatrix} a & b \\ c & d \\ \end{bmatrix} and matrix B = [ c a d b ] B = \begin{bmatrix} c & -a \\ d & b \\ \end{bmatrix} .

Then multiplying these matrices, we will obtain: A × B = [ a b c d ] [ c a d b ] = [ a c + b d b 2 a 2 c 2 + d 2 b d a c ] A\times B = \begin{bmatrix} a & b \\ c & d \\ \end{bmatrix} \cdot \begin{bmatrix} c & -a \\ d & b \\ \end{bmatrix} = \begin{bmatrix} ac+bd & b^2 - a^2 \\ c^2 + d^2 & bd - ac \\ \end{bmatrix}

Calculating the Determinants of these matrices, we know that the det ( A B ) = det ( A ) det ( B ) \text{det}(AB) = \text{det}(A) \cdot \text{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 ) (ad-bc)(ad+bc) = (ac+bd)(bd-ac) - (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 ) = 341 + 5 × 74 = 370 341 = 29 (ad)^2 - (bc)^2 = [(bd)^2 - (ac)^2] + (a^2 - b^2)(c^2 + d^2) = -341 + 5\times 74 = 370 - 341 = \boxed{29} .

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 (a^2 - b^2)(c^2+d^2) = (ac)^2 - (bd)^2 + (ad)^2 - (bc)^2 directly.

To me, representing it in matrix form obscures what is going on.

Calvin Lin Staff - 4 years, 6 months ago

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Well, in a way. I just wanted a problem with matrix determinant and just came up with this.

Worranat Pakornrat - 4 years, 6 months ago

Similar to Lagrange's identity!

Nihar Mahajan - 4 years, 6 months ago

Using matrices looks much more complicated but, on the other hand, gives a more generalizing approach to problem. That's my take.

Thái An Lê - 1 year, 3 months ago

Just multiplied the first 2 equations and that's it.

Shreyash Rai - 4 years, 6 months ago

(+-3,+-2,+-7,+-5)

Saya Suka - 4 years, 6 months ago
Fidel Simanjuntak
Dec 13, 2016

Yes, we can factorize those equations.

a ² b ² = 5 a² - b² = 5

( a + b ) ( a b ) = 5 × 1 (a+b)(a-b) = 5 \times 1

a + b = 5 a+b = 5 ; a b = 1 a-b=1

Solving, we get a = 3 a=3 and b = 2 b=2 .

( a c ) ² ( b d ) ² = 341 (ac)² - (bd)² = 341

( 3 c ) ² ( 2 d ) ² = 341 (3c)² - (2d)² = 341

( 3 c + 2 b ) ( 3 c 2 b ) = 341 (3c+2b)(3c-2b) = 341

Since 3 c + 2 b > 3 c 2 b 3c+2b > 3c-2b , then we have

3 c + 2 b = 341 3c+2b = 341 or 31 31 ; and 3 c 2 b = 1 3c-2b = 1 or 11 11

First condition. If 3 c + 2 b = 341 3c+2b = 341 and 3 c 2 b = 1 3c-2b =1 ,

then c ² + d ² 74 c² + d² \neq 74 .

Second condition; 3 c + 2 b = 31 3c +2b =31 and 3 c 2 b = 11 3c-2b = 11 , solving we get c = 7 c=7 and d = 5 d=5 .

Then, the second equation is completed.

Now,

( a d ) ² ( b c ) ² = ( 3 × 5 ) ² ( 2 × 7 ) ² (ad)² - (bc)² = (3 \times 5)² - (2 \times 7)²

= ( 15 + 14 ) ( 15 14 ) = 29 × 1 = (15+14)(15-14) = 29 \times 1

= 29 = \boxed{29}

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 a^2,b^2,c^2,d^2 . So you can also solve by setting b = 0 b=0 :

a 2 = 5 a^2=5

5 c 2 = 341 = > c 2 = 68.2 5c^2=341 => c^2=68.2

c 2 + d 2 = 74 = > d 2 = 5.8 c^2+d^2=74 => d^2=5.8

( a d ) 2 ( b c ) 2 = 5 d 2 = 29 (ad)^2-(bc)^2 = 5d^2 = 29

Alex Burgess - 2 years, 2 months ago

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