# Math Help - Set of 4 simultaneous equations using modular

1. ## Set of 4 simultaneous equations using modular

Hi all,

I have the following set of equations:

(13a + 4b)mod729 = 19
(13c + 4d)mod729 = 15
(4a + 3b)mod729 = 5
(4c + 3d)mod729 = 3
(11a + 4b)mod729 = 5
(11c + 4d)mod729 = 7

I feel a lot more comfortable solving this if i had 2 unknowns. With 4 unknowns do i need to create a matrix or anything to solve them?

Thanks!

2. Originally Posted by sirellwood
Hi all,

I have the following set of equations:

(13a + 4b)mod729 = 19
(13c + 4d)mod729 = 15
(4a + 3b)mod729 = 5
(4c + 3d)mod729 = 3
(11a + 4b)mod729 = 5
(11c + 4d)mod729 = 7

I feel a lot more comfortable solving this if i had 2 unknowns. With 4 unknowns do i need to create a matrix or anything to solve them?

Thanks!
I don't think that there is a solution to this. Consider the following two equations:
$13a + 4b = 19$

and
$4a + 3b = 5$

We should be able to solve this system. So from the second equation we get
$a = 4^{-1} \cdot (5 - 3b)$

Mind you this is in mod 729. So we need the multiplicative inverse of 4 in mod 729. This is 547. Thus
$a = 547 \cdot ( 5 - 3b) = 548 - 183b$

Put this value for a in the first equation:
$13(548 - 183b) + 4b= 19$

etc, etc. I get a = 65 and b = 158. But

$11a + 4b = 618$
which is not 5 as your equations specifiy.

-Dan

3. Thanks topdsquark... you are right. I made an error when forming my equations. Basially i am solving some cipher matrices and i thought this one was linear when it is infact affine so ill have to redetermine my equations and post a new thread!

4. Hello, sirellwood!

Is there a typo? .The system is inconsistent.

I have the following set of equations:

. . $\begin{array}{cccccccc}
\,[1] &13a + 4b &\equiv& 19 &\text{(mod 729} \\
\,[2] & 13c + 4d &\equiv& 15 &\text{(mod 729)} \\
\,[3] & 4a + 3b &\equiv& 5 & \text{(mod 729)} \\
\,[4] & 4c + 3d &\equiv& 3 & \text{(mod 729)} \\
\,[5] & 11a + 4b &\equiv& 5 & \text{(mod 729)} \\
\,[6] & 11c + 4d &\equiv& 7 & \text{(mod 729)}
\end{array}$

Look again . . . You don't have to solve for four unknowns all at one time.

[1] and [3] comprise a system you should be able to solve.

. . $\begin{array}{ccccccc}
\,[1] & 13a + 4b &\equiv& 19\text{ (mod 729)} \\
\,[3] & 4a + 3b &\equiv& 5 \text{ (mod 729)} \end{array}$
. .
. . . I'll drop the (mod 729) for now.

$\begin{array}{cccccccc}
\text{Multiply [1] by 3:} & 39a + 12b &\equiv& 57 \\
\text{Multiply [3] by 4:} & 16a + 12b &\equiv& 20 \end{array}$

$\text{Subtract: }\:23a \:\equiv\:37$

$\text{Multiply by 317: }\;317(23a) \:\equiv\;317(37)$

. . . . . . . . . . . . . . . $7291a \:\equiv\:11,\!729$

$\text{Since }\,7291 \,\equiv\,1\text{ (mod 729)}\:\text{ and }\:11,\!729\,\equiv\,65\text{ (mod 729)}$

. . $\text{we have: }\:\boxed{a \:\equiv\:65\text{ (mod 729)}}$

Substitute into [3]: . $4(65) + 3b \:\equiv\:5 \quad\Rightarrow\quad 260 + 3b \:\equiv\;5$

. . $3b \:\equiv\:-255 \:\equiv\:474 \quad\Rightarrow\quad \boxed{b \:\equiv\:158 \text{ (mod 729)}}$

Do the same for [2] and [4].

You will find that: . $\boxed{c\,\equiv\,255,\;d\,\equiv\,633\text{ (mod 729)}}$

But we find that these answers do not satisfy [5] and [6] . . .

5. The OP has said in a private communication that the equations were wrong. However, I believe the help given in this thread should be sufficient for the OP to attempt to solve the correct equations.

6. thanks soroban. yeah as i posted above, this question was to do with an affine cipher matrix, not a linear cipher matrix so my set of equations should look like this i think....

(13a + 4b)mod729 = 19-e
(13c + 4d)mod729 = 15-f
(4a + 3b)mod729 = 5-e
(4c + 3d)mod729 = 3-f
(11a + 4b)mod729 = 5-e
(11c + 4d)mod729 = 7-f

Does it work now?

7. and yes mrfantastic, the help was very useful as i came out with

a=7, b=680 c=4, d=705, e=124 and f=59....for the record

8. Sorry to keep going back to this thread but has anyone else tried to solve it and got the same numbers as me? I ask because the numbers form a 2x2 and 2x1 matrix that are needed to do something else and although my answers seem to hold fine in the first step, when i use them in the second, they do not anymore.

9. Originally Posted by sirellwood
Sorry to keep going back to this thread but has anyone else tried to solve it and got the same numbers as me? I ask because the numbers form a 2x2 and 2x1 matrix that are needed to do something else and although my answers seem to hold fine in the first step, when i use them in the second, they do not anymore.
There may be more general solutions lurking out there, such as a = 7 + 35n or something. But I ran your numbers through the equations and they check out.

-Dan

10. ok, so as long as all the solutions are mod729, does that mean i take my answers to be correct?