Question

find the sum of the following arithmetic progression:-

1) 9,12,15,...........up to 16th terms .
2) 8,3,-2,.................up to 22 terms.
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Answer 1

Sn = n/2(2a +( n-1)d)

S16 = 16/2(2(9) + (16-1) 3)

= 8(18 +(15)3)

= 8(18+45)

= 8(63)

Sn = 504

S22 = 22/2 (2(8)+(22-1)-5)

= 11 (16 + (21)-5)

= 11(16-315)

= 11(-229)

= - 3289

hope it help you