Question

find the sum of the first 15 terms of the following ap an= 2-3n

Answer 1

Answer:

First term, a=1,15

th

term a

15

=3−30=−27

Sum of 15 terms =

2

n

(a+l(lastterm))=

2

15

(1−27)=15(−13)=−195

Answer 2

First term, a = 1

15th term, a15 = 2-3n

= 2-3(15)

= 2-45

= -43

$Sum \: of \: 15th \: term = \frac{n}{2}(a + l) \\ \: \: \: \: \: \: \: \: \: \: \: \: \: = \frac{15}{2} (1 - ( - 43))$

$\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: = \frac{15}{2} \times 44 \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: = \: \: 15 \times 11 \\ \: \: \: \: \: \: \: \: = 165$

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