# last two digits of $9^{1500}$ (Dummit Foote -Abstract Algebra preliminaries $0.3.5$)

Question is to find last two digits of $9^{1500}$ (No Euler totient theorem please)

What i have done so far is :

$9^2\equiv 81\pmod{100}$

$9^4 \equiv 61\pmod{100}$

$9^8\equiv 21\pmod{100}$

$9^{16} \equiv 41\pmod{100}$

$9^{32} \equiv 81\pmod{100}$

higher powers of $9$ namely $64,128,256,512,1024$ will be in repeated pattern as above.

$9^{64} \equiv 61\pmod{100}$

$9^{128} \equiv 21\pmod{100}$

$9^{256} \equiv 41\pmod{100}$

$9^{512} \equiv 81\pmod{100}$

$9^{1024} \equiv 61\pmod{100}$

Now,I want to split power of $9$ i.e., $1500$ to powers which i have noted down above. i.e,

$9^{1500}=9^{1024}.9^{476}$

$9^{1500}=9^{1024}.9^{256}.9^{220}$

$9^{1500}=9^{1024}.9^{256}.9^{128}.9^{92}$

$9^{1500}=9^{1024}.9^{256}.9^{128}.9^{64}.9^{28}$

$9^{1500}=9^{1024}.9^{256}.9^{128}.9^{64}.9^{16}.9^{12}$

$9^{1500}=9^{1024}.9^{256}.9^{128}.9^{64}.9^{16}.9^{8}.9^4$

When you multiply two positive integers, the last digit in the product depends on those two integers only through their last digits.

So, I will look only for last two digits of $9$ in above powers.

$9^{1500}=9^{1024}.9^{256}.9^{128}.9^{64}.9^{16}.9^{8}.9^4$

$\equiv 61.41.21.61.41.21.61 \pmod{100}$

$\equiv (61.61).(21.21).(41.41).61\pmod{100}$

(we have already seen above $61.61\equiv 21 \text{mod}100$ and similarly for other cases). So, we would be left with :

$\equiv (21).(41).(81).(61)\pmod{100}$

$\equiv (61)(41)\pmod{100}$

$\equiv (01) \pmod{100}$ I would be happy if someone can verify the procedure I have done and I would be thankful if some one can help me to make this less laborious and more efficient.

• Is it $1000$ or $1500$ in the exponent? – TMM Oct 26 '13 at 13:13
• Thank you for pointing out... Error Edited :) – user87543 Oct 26 '13 at 13:15
• You could also use the binomial theorem, but I don't know if you consider that as basic. – Ivan Loh Oct 26 '13 at 13:23
• @IvanLoh : Thanks for pointing out another possible way.. I have not yet worked out in that way.. I do consider it as basic :) I will try working it out.. Thank You.. – user87543 Oct 26 '13 at 13:26

You have seen that $9^2 \equiv 9^{32} \equiv 81$ mod $100$. As gcd$(81,100) = 1$, this implies $9^{30} \equiv \frac{9^{32}}{9^2} \equiv \frac{81}{81} \equiv 1$, and thus $9^{1500} = (9^{30})^{50} \equiv 1$.

Your method also works, but it is longer.

• This does makes perfect sense and Thank You :) – user87543 Oct 26 '13 at 13:22

$$\phi(100)=\phi(5^2)\phi(2^2)=40$$ where $\phi$ is Euler's Totient function. Then $$9^{1500}=9^{20}9^{37*40}=9^{20}=3^{40}=1$$ Using Fermat's little theorem.

Given the answer above by Arthur, I still would like to add my shorter version.

1: $9^5\equiv49\pmod{100}$

$$9^5=59049$$

2: $9^{10}\equiv1\pmod{100}$

$$9^{10}\equiv49\times49=2401\equiv1\pmod{100}$$

3: $$9^{1500}=(9^{10})^{150}\equiv1\pmod{100}$$

A similar question has been asked a while back. The whole idea is to exploit the fact that
$9^n=(10-1)^n$ and/or the fact that $9^{2n}=81^n=(8\cdot10+1)^n$, and plug it into Newton's
Binomial Theorem. It will soon become evident that our number is of the form $\mathcal{M}_{100}+1$.

I saw the remaining answers, but I don't think that so much work is required. $$9^{1500} = (10-1)^{1500} = 1 - 15000 + 10k = 1 + 10k'$$ where $$k,k'\in\mathbb{Z}$$, by binomial theorem.

So we have $$9^{1500} \equiv 1 + 10k' \bmod 100$$ $$9^{1500} \equiv 1 \bmod 100$$ which is what we want!