Question

3.8.15.24.n 求第n个数是几

Answer 1

a1=3 a2=8 a3=15 a4=24

a2-a1=8-3=5=2*2+1

a3-a2=15-8=7=2*3+1

a4-a3=24-15=9=2*4+1

.

an-a(n-1)=2*n+1

相加得

an-a(n-1)+.+a4-a3+a3-a2+a2-a1=2*(n+.4+3+2)+n-1

an-a1=2(n+2)(n-1)/2+n-1=n^2+n-2+n-1=n^2+2n-3

an=n^2+2n-3+a1=n^2+2n-3+3=n^2+2n