Question

the common difference of an AP is -2 find its sum if first term is 100 and the last term is -10​

Answer 1

Step-by-step explanation:

d=-2

a=100

an=-10

sn=?

n=?

an=a+(n-1)d

-10=100+(n-1)-2

-10-100=(n-1)-2

-110=(n-1)-2

-110/-2=n-1

55=n-1

56=n

sn=n/2(a+an)

=56/2(100-10)

=28(90)

sn=2520