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bmwhype2
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sequence and units digit 13 Oct 2007, 22:15
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is there a different way to solve this question without going thru the tedious way MGMAT explained it?


If Sn = 4^n + 5 ^(n+1) + 3, what is S100?



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GMAT 1: 680 Q48 V35
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sequence and units digit 13 Oct 2007, 22:20
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bmwhype2
is there a different way to solve this question without going thru the tedious way MGMAT explained it?


If Sn = 4^n + 5 ^(n+1) + 3, what is S100?
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What are the options here ?
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bmwhype2
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sequence and units digit 13 Oct 2007, 22:22
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ashkrs
bmwhype2
is there a different way to solve this question without going thru the tedious way MGMAT explained it?


If Sn = 4^n + 5 ^(n+1) + 3, what is S100?

What are the options here ?
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It was an open ended question at the end of the chapter. no choices.
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sequence and units digit 16 Oct 2007, 07:53
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bmwhype2
is there a different way to solve this question without going thru the tedious way MGMAT explained it?


If Sn = 4^n + 5 ^(n+1) + 3, what is S100?
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so whats MGMAT's way?
or if there is an easy way, can someone explain?
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yuefei
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sequence and units digit 16 Oct 2007, 10:43
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If Sn = 4^n + 5 ^(n+1) + 3, what is S100?
-------------------
More than likely the answers would be in exponential form.

S(100) = 4^100 + 5^100 + 3
= 2*2*100 + 5^100 + 3 = 2^101 + 5^101 +3
Since the exponent is the same, multiply the bases together:
= (2*5)^101 + 3

= 10^101 + 3
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sequence and units digit 16 Oct 2007, 10:56
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quick fix for such problems...

for a number ending with 0,1,5,6 ..... any power will be 0,1,5,6

for numbers 4 and 9 - you have odd even formula
for 4 - odd power- unit digit - 4
for 4 - even power- unit digit - 6
for 9 - odd power- unit digit - 9
for 9 - even power- unit digit - 1

for rest ie 2,3,7,8 it is cycle of 4(divide the power by 4)

for 2 - 2,4,8,6
for 3 - 3,9,7,1
for 7 - 7,9,8,6
for 8 - 8,4,2,6

for example 8^5 - divide 21/4 = reminder 1 terefore unit digit is 8.

try this out and lemme know

yu guys owe me ;)



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Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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Thank you for understanding, and happy exploring!