How would one use modular arithmetic to find the units digit of 7^7^7?

tens digit of 2^65 ?

How would one use modular arithmetic to find the units digit of 7^7^7?

tens digit of 2^65 ?

No vote yet

3 votes

×

Problem Loading...

Note Loading...

Set Loading...

## Comments

Sort by:

TopNewestFOR TENS DIGIT OF 2^65, FIND ITS MOD 100.

NOTE THAT 2^10=1024 = 24 (MOD 100) sO 2^20 = 576(MOD 100) = -24(MOD100) 2^40= (-24)^2 (MOD 100) = -24 (MOD100) AGAIN! 2^60 = 2^20 *2^40 (MOD 100) = 576(MOD100) = -24(MOD100)

ALSO, 2^5 = 32(MOD100) SO 2^65=2^60 * 2^5(MOD 100) = (-24)(32)(MOD100) = 32(MOD100)

SO THE TENS DIGIT IS 3 AND THE UNITS DIGIT IS 2. – Shourya Pandey · 4 years, 5 months ago

Log in to reply

The first problem :

\(7^{7^{7}} \equiv 7^{3} \equiv 3 \pmod {10}\) and hence the unit digit is \(3\). (The powers of \(7\) form a cycle mod \(10\). ) – Zi Song Yeoh · 4 years, 5 months ago

Log in to reply

– Namra Aziz · 4 years, 5 months ago

how can you equate power 7 as power 3. I guess this is wrong. Please clarify.Log in to reply

– Zi Song Yeoh · 4 years, 5 months ago

\(7^{7} \equiv 7^{3}\cdot7^{4} \equiv 7^{3} \equiv 3 \pmod{10}\). Since \(7^2 \equiv -1 \pmod {10} \Rightarrow 7^{4} \equiv 7^{2}\cdot7^{2} \equiv 1 \pmod{10}\)Log in to reply

– Harshit Kapur · 4 years, 5 months ago

Just use euler's theorum for \(7^4\), since phi(10) = 4Log in to reply

– Zi Song Yeoh · 4 years, 5 months ago

Yes.Log in to reply

– Nishanth Hegde · 4 years, 5 months ago

no..its perfectly alright..Log in to reply

– Ram Prakash Patel · 4 years, 5 months ago

But unit digit would be 7.Log in to reply

– Rohan Rao · 4 years, 5 months ago

Zi Song is right. The answer is 3.Log in to reply