Question

Find 60th term of an AP 8,10,12.....if it has total of 60terms and hence find the som of last 10 term?

Answer 1

Solution :

Given A.P=8,10,12....

n=60

a=8

d=2

an=a+(n−1)d

a60=8+59×2

=126

Hence sum of the last 10 items is

d=−2,a=126,r=10

s=n/2[2a+(n−1)d]

S=10/2[2×126+(10−1)(−2)]

=5(252−18)

=1170

Answer 2

First term = a = 8

Common difference = d = 10 - 8 = 2

No : of terms = n = 60

60th term  = a₆₀ = a + 59d = 8 + 59*2 = 126

60th term is 126

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Sum of last ten terms ,

If we consider the last 10 terms , as A.P , then

Common difference = d = -2

First term = a₆₀ = 126

no: of terms = n = 10

Applying Sn formula :-

Sn = n/2*( 2 a + (n-1)d)
     = 10/2* ( 252 + 9*-2)
     = 5*(234)
     = 1170

Sum of last 10 terms = 1170