Question

the first term of an A.P is 1 and the nth term is 25. if Sn is 520, find n

Answer 1

Answer:

n = 40

Step-by-step explanation:

Given, the first term (a) = 1,  $a_{n}$= 25 and  $S_n$= 520

We know that

$S_n$ = $n\frac{(a + a_n)}{2}$, where n = number of terms

=> 520 = n(1 + 25)/2

=> 520 = n(26/2)

=> 520 = 13n

=> n = 520/13

=> n = 40

Answer 2

Answer:

n=40

Step-by-step explanation:

given, a=1

          aₙ=25=l

          Sₙ=520

now, Sₙ = 520

∴ (n/2)(a+l)=520

∴(n/2)(1+25)=520

∴n=(520×2)/26

∴n=40