Hi,

please could someone assist with the following:

This is the pattern give:

Pattern number 1 2 3 4

Number of dots 2 6 12 20

The question is: Find s formula for the general term

Their solution goes as follows:

The second difference is constant therefore Tn is quadratic - I understand this

therefore Tn = an2+bn+c this i understand

but 2a=a - I cannot understand the basis of this statement

Can anyone enlighten me?

Thanks!