Question

Find the sum of the following series

i. - 15 - 10-5-0+ ... +70.
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Answer 1

Answer:

495

MARK IT AS BRAINLIEST

Step-by-step explanation:

last term is (an)= 70

first term is (a)= -15

common difference is (d)= 5

to find (Sn)=?

so

an=a+(n-1)d

70=-15+(n-1)5

70+15=(n-1)5

85/5=n-1

17+1= n

n=18

Now

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

Sn=18/2*[2<-15>+(18-1)5]

Sn=9*[-30+85]

Sn=9*(55)

Sn=495