Question

common difference of an ap is -2 and the first term is 80 . find the sum if last term is 10​

Answer 1

Answer:

Given,

d = -2

a = 80

l = 10

To find,

Sum of n terms.

Solution,

we know,

l = a + (n - 1)d

putting all the given values,

10 = 80 + (n - 1)(-2)

10 - 80 = - 2 (n -1)

- 70 = -2(n -1)

70/2 = (n - 1)

35 = n - 1

36 = n

Now,

Sn = n/2 (a + l)

S = 36/2 (80 + 10)

S = 18 × 90 = 1620

Hence, Sum of all the terms of the A.P is 1620.

Answer 2

Answer:

here, Common difference (d) =-2

the first term(a) =80

the last term(n) =10

sum of the terms(Sn)=n/2[2a+(n-1) d]

=10/2[2×80+(10-1) ×(-2)

=5[160+9×(-2)]

=5(160-18)

=5×142

=710