Question

Find the sum of 15 terms of the arithmetic series 25+19 + … .

Answer 1

Answer:S15= -255

Step-by-step explanation:

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

= 15/2[2×25+(15-1)×-6]

=15/2[50+14×-6]

=15/2[50-84]

=15/2×-34

=15×-17

=-255