There is a2=a 1+d,

a3 = a2+d = a 1+d+d = a 1+2d，

a4=a3+d=a 1+3d，

Through observation, the nth term is the first term plus (n- 1) multiplied by the tolerance d,

That is, an = a1+(n-1) × d.

This conclusion can be drawn through students' observation.