等差数列解答题在等差数列an中,a5=0.3,a12=3.1,求a18+a19+a20+a21+a22的值

a5=a1+4d=0.3 (1)
a7=a1+11d=3.1 (2)
(2)×15-(1)×8
7a1+133d=44.1
a1+19d=6.3
a18+a22=a19+a21=2a20
所以原式=5a20
=5(a1+19d)
=31.5