等差数列｛an｝中,a10=30,a20=50,则通式an=
问题描述：
等差数列｛an｝中,a10=30,a20=50,则通式an=
答
a10=a1+9d
a20=a1+19d
d=(50-30）/(19-9)
=2
a1=a10-9d
=30-18
=12
an=a1+(n-1)d
=12+2n-2
=2n+10