How many different ways can a director select 4 actors from a group of 20 actors?
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ANSWER
EXPLANATION
The given sequence is
3,-5,-13,.....-229.
The first term of this sequence is
a_1=3
There is a common difference of
The last term of this sequence is -229.
The nth term of this sequence is given by:
We use the last term to determine the number of terms in the sequence.
Divide through by -8;
Hence there are thirty terms in the sequence.
The sum of the first n terms is given by:
The sum of the first 30 terms is given by: