Question

10,15,21,28,36,45,55.规律公式

Answer 1

a1=10,

a2-a1=5,

a3-a2=6

a4-a3=7

a5-a4=8

an-a(n-1)=3+n

an-a1=5+6+7+...(n+3) = (n+8)*(n-1)/2

an=10+(n^2+7n-8)/2= (n^2+7n+12)/2=(n+3)(n+4)/2