8. The number of shaded squares shown in the patterns below forms the first three terms of an arithmetic sequence.

The first image is a cross and there are the four shaded squares - n=1

The second image shown looks like the first image except that three more squares are shaded making it look like two crosses. There are seven shaded squares - n=2

In the third image, there are three more shaded than the last time making it seem to be three crosses. There are 10 squares shaded - n=3

Determine the number of squares that would be shaded in the 20th pattern.

9. Kirk believes the formula a2= 3n+4 will predict the correct number of shaded squares in the pattern above. Explain why Kirk is incorrect.