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To get the first term, simply substitute n= 1 into the formula and then calculate
f(1) = 4*2(1-1) = 4*2*0 = 0.
To get the second term, simply substitute n= 2 into the formula and then calculate
f(2) = 4*2(2-1) = 4**2*1 = 8.
I leave it to you to do the same for the third and the fourth terms on your own.
There are NOTHING easier than that.
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By the way, what you wrote in your post IS NOT a geometric progression.
To show a geometric progression, the formula should be f(n) = 8*2^(n-1).
Learn how to write formulas correctly.