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In the sequence 1,2,2,4,8,32,256...

each term (starting from the third term) is the product of the two terms before it. For example, the seventh term is 256  , which is the product of the fifth term (8) and the sixth term (32).

This sequence can be continued forever, though the numbers very quickly grow enormous! (For example, the 14'th term is close to some estimates of the number of particles in the observable universe.)

What is the last digit of the  term of the sequence

 

a1=1a2=2a3=a2a1=a12a4=a3a2=a12a2=a22a5=a4a3=a22a12=a32a6=a5a4=a32a32=a52a7=a6a5=a52a32=a82a8=a7a6=a82a52=a132a9=a8a7=a132a82=a212

Let F are the Fibonacci number F1=1F2=1F3=2F4=3F5=4F6=5F7=13F8=21F9=34F10=55F11=89F12=144F13=233F14=377

an=aFn12|a2=2an=2Fn1

 

If n = 14:

a14=2F141a14=2F13|F13=233a14=2233 The last digit of the term a142233(mod10)21317+12(mod10)(213)17212(mod10)|2132(mod10)217212(mod10)229(mod10)2132+3(mod10)(213)223(mod10)|2132(mod10)2223(mod10)25(mod10)32(mod10)2(mod10)

 

 The last digit of the term a14 is 2

 

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Jan 21, 2019
Jan 20, 2019

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