Question

1.在等差数列+(a_n)+中,已知+a_3=21+,+a_8=36+求通项公式an和第25项。

Answer 1

an = a1+(n-1)d
a3=21
a1+2d=21 (1)
a8=36
a1+7d=36 (2)
(2)-(1)
5d=15
d=3
from (1)
a1+2d=21
a1+6=21
a1=15
an =15+3(n-1) = 3n+12
a25 =3(25)+12=87