hi folks,

I hope this is the right place to post this! I have no idea how to proceeed with the following question:

Show that for all positive integral values of n

[HTML]7 <sup>n</sup> + 2 <sup>2n+1</sup>[/HTML]

is divisible by 3.

I tried a few terms as follows:

n = 1. 7 ^ 1 + 2 ^ 3 = 7 + 8 = 15

n = 2. 7 ^ 2 + 2 ^ 5 = 49 + 32 = 81

n = 3. 7 ^ 3 + 2 ^ 7 = 343 + 128 = 471

and these are all divisible by 3 but how do I handle the general case?

I thought of expanding the expression i.e.

2.2 ^ n. 2 ^ n - (1 - 8) ^ n

and using a binomial on the second term but it doesn't get me anywhere. I guess I am trying to calculate the sum of a series and show that it has a factor of 3 but I can't see how to do it. Any ideas?

regards and thanks

Simon

p.s. sorry about the formating. the HTML stuff doesn't seem to come out so I resorted to the ^ symbol which is pretty unpretty!