Question

in an ap if the 6th and 13th terms are 35and70 . find the sum of its first 20 terms

Answer 1

may this help you .
thanks

Answer 2

given - the 6th term of ap = 35

13 term = 70

to find - sum of first 20 terms

35 = a+34d 1...

70 = a+69d. 2...

by subtracting 1 and 2

35- 70=a+34d-{a+69d}

-35=a+34d-a-69d

-35=34d-64d. (a canceled out)

-35=-35d

35d=35d. ( minus cancelled )

d=35/35

d=1

put.in any of the equation 1&2

35=a+34d

35=a+34(1)

35=a+34

35-34=a

1=a

S=n/2 [2a+(n-1) d]

S=20/2 [ 2×1 +(20-1)1 ]

S=10[2+19]

S=10×21

S=210