Question

1\n(n-2)+1\(n-2)(n-4)••••••+1\(n-(n-3))(n-(n-1))=9\20,n=____

Answer 1

1/{n(n-2)} + 1/{(n-2)(n-4)} + •••••• + 1/(3*1) = 9/20

2/{n(n-2)} + 2/{(n-2)(n-4)} + •••••• + 2/(3*1) = 9/10

1/(n-2) - 1/n + 1/(n-4) - 1/(n-2) + •••••• + 1/1 - 1/3 = 9/10

1 - 1/n = 9/10

1/n = 1 - 9/10 = 1/10

n = 10