Find $9^A$ mod 100 Given $9^{40}$ mod 100 = 1

  • MHB
  • Thread starter Albert1
  • Start date
In summary, the value of A in the equation $9^A$ mod 100 can be found by taking the logarithm of both sides. A calculator can be used to solve this equation. Alternatively, the properties of modular arithmetic can be used to simplify the equation and find the value of A faster. In cases where A is not an integer, the Chinese Remainder Theorem can be used to find a unique solution for A mod 100. This method can also be applied to other modular equations, but it is important to consider the properties of modular arithmetic before using it.
  • #1
Albert1
1,221
0
given $9^{40}$ mod 100=1

if $A=9^9$

find $9^A $ mod $100=?$
 
Mathematics news on Phys.org
  • #2
Albert said:
given $9^{40}$ mod 100=1

if $A=9^9$

find $9^A $ mod $100=?$
we can compute
A = 387420489
so A mod 40 = 9
so $9^A$ mod $100$ =$9^9$ mod 100 = 387420489 mod 100 = 89
but it is no fun
we have 40 = 8 * 5
9 mod 8 = 1
so $9^9$ mod 8 = 1
9 mod 5 = -1
so $9^9$ mod 5 = -1
we can now find $9^9$ mod 40 by chinese remainder theorem by by taking multiples of 8(only 5 multiples) and adding 1 we get checking as -1 mod 5
$9^9$ mod 40 = 9
hence A mod 40 = 9
now we need to find
$9^A$ mod 100 = $9^9$ mod 100
so let us find mod 4 and mod 25
$9$ mod 4 = 1
so $9^9$ mod 4 = 1

$9^3 = 729 = 4$ mod 25
so $9^9 = 4^3 = 64 =14 $ mod 25
we take 25 n+ 14 and divide by 4 to see remainder 1 the values are 14, 39, 64, 89 and see the result 89
 

FAQ: Find $9^A$ mod 100 Given $9^{40}$ mod 100 = 1

How can I find the value of A in the equation $9^A$ mod 100 given that $9^{40}$ mod 100 = 1?

The value of A can be found by taking the logarithm of both sides of the equation. This will give us the equation A*log(9) = log(1) = 0. Since log(9) = 2, A must be equal to 0.

Can I use a calculator to solve this equation?

Yes, you can use a calculator to solve this equation. Just remember to use the logarithm function to find the value of A.

Is there a faster way to solve this equation?

Yes, there is a faster way to solve this equation. You can use the properties of modular arithmetic to simplify the equation. For example, since 9 and 100 are relatively prime, we can use Euler's theorem to rewrite the equation as $9^{\phi(100)}$ mod 100 = 1, where $\phi$ is the Euler totient function. Since $\phi(100) = 40$, we can see that A must be equal to 1.

What if the value of A is not an integer?

In this case, we can still use the properties of modular arithmetic to find the value of A. For example, if A is a fraction, we can use the Chinese Remainder Theorem to find the value of A mod 100. This will give us a unique solution for A.

Can I use this method to solve other modular equations?

Yes, this method can be applied to other modular equations as well. However, it may not always give a unique solution for A. In some cases, there may be multiple solutions or no solutions at all. It is important to check the properties of modular arithmetic before using this method to solve an equation.

Back
Top