Question

If the second and fifth terms of an AP are 1 and 22, then the sum of first 15 terms is

Answer 1

Answer:

hey mate here is ur ans

Step-by-step explanation:

an = a+(n-1) d

a2 = a+(2-1) d = a+(1)d = 1 ---->1

a5 = a+(4d) = 22 ------>2

from 1 and 2

3d = 21

d = 21/3 = 7-------->3

from 3 and 1

a+1d= 1

a= 1-7 = -6

so ,a15 = a+14d = -6+14(7) = 92

hope it help u

Answer 2

Answer:

Given: second term of the AP = 1

Fifth term of that AP=22

Solution: General term of an AP is :-

an = a+(n-1) d

According to the given conditions,

a2 = a+(2-1) d = a+(1)d = 1 ..........i)

a5 = a+(4d) = 22 .........ii)

from i) and ii)

3d = 21

d = 21/3 = 7.............. iii)

from iii) and i)

a+1d= 1

a= 1-7 = -6............. iv)

Sn=n/2 (2a+(n-1)d)

substituting the above values, we get

:. S15 = 15/2{2(-6) + (14)d}

= 15/2 {-12+ 98}

= 15 / 2 {86}

= 15x43

= 645

:. Sum of First 15 terms of that AP is 645