Question

given a=7 a13 =35 find d and s13

Answer 1

a13=a+12d =35. 7+12d. =35. 12d =35-7. 12d=28. d=28/12.=7/3. S13 =13/2(7+35) =13×42/2=13×21=273. {Sn= n/2[a+an]}

Answer 2

Answer:

d = 7/3, Sn=273

Step-by-step explanation:

First term of an AP = a = 7

Thirteenth term of an AP = 35

a + 12d = 35 ------(1)

Substitute a in eq - (1)

a + 12d = 35

(7) + 12d = 35

12d = 35 - 7

12d = 28

d = 28/12

d = 7/3

In an AP sum of the terms = n/2 ( a + an )

= 13/2 ( 7 + 35)

= 13/2 ( 42)

= 13(21)

= 273