If geometric sequence = a1 = 3 a7 = 2187 and r = 3 find S7
in this Qustion which formula we use
I use this : Sn = a1(1-r^n ) / 1- r
the answer was = - 39
please help me ..
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If r < 1 or r > 1
also we use same formula
Yes, that's true. If r= 1, then the sum is na.
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