Question

in an sn=256 a=1 and d=2find n

Answer 1

Use formula Sn=n÷2 (2a+(n-1)d)
256×2=n (2+2n-2)
512÷2=n×n
n×n=256
n=16

Answer 2

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

256=n/2{2+(n-1)2}

256=n/2{2+2n-2}
+2and-2 are cancel
256=n/2×2n
256×2=n×2n
512=2n^2
divide by 2
256=n^2
n^2=√256
n=16