The sum of the first 21 terms of the progression _18,_18,_12 is 441
I need the answer of the first question
but you asked no question !
You also did not say if the progression is arithmetic or geometric.
Perhaps you are looking for the missing terms??
ok then.....
looks like term(2) = 18 and term(4) = 18
If GP:
ar = 18
ar^3 = 18
divide them:
r^2 = 1
r = ± 1
if r = 1, your sequence would have all its terms alike, but it looks like 12 messes that up, so you don't have a GP
if AP:
a+d = 18
a+3d = 18
subtract them:
2d = 0
d = 0
so the common difference would be 0, and all the terms would be
the same.
Same problem!
Your problem is bogus.