Question

if the first term of an ap is 7 and 13th term is 35 find the sum of 13 term of the sequence​

Answer 1

a=7

t13 = 35

t13=a+(n-1)d

35=7+(13 -1)d

35-7=12d

28=12d

28/12=d

d=2.67

sn=n/2{2a +(n-1)d}

sn=13/2{(2×7)+(13-1)2.67

sn=13/2{14+(12×2.67)}

sn=13/2{14+32.04}

sn=13/2{46.04}

sn=208.26.....Ans:-

Answer 2

Answer:

Step-by-step explanation:

First we need to find the common difference

a1=7

a13=35

So we know that nth term of AP is given by a1+(n-1)d

So 13th term is given by

a13=a1+12d=7+12d=35.

Now d=28/12.

Sum to n terms of AP is given by

Sn=n/2 * (2a1+(n-1)d)

S13=13/2 * (2*7+(13-1)(28/12))

=13/2* (14+28)

=13/2* (42)

=13*21

=273 which is the correct answer for sum upto 13th term of this AP:))