等差數列及等比數列

sum of first 10 terms = 10^2 = 100
sum of first 9 terms = 9^2 = 81
so the 10th term = 10^2 - 9^2 = 19
So answer is A
No need to find first term and common difference!!!

某等差數列的首n項和為n^2,求該數列的第10項。

設首項是a﹐公差是d

首n項和=n/2[2a+(n-1)d]=n^2

代n=1

a=1

代n=2

2a+d=4

d=2

第10項

=a+(10-1)d

=a+9d

=1+9*2

=19

答案是a