by **Animator** » Fri May 20, 2005 7:23 pm

Well,

First, let us look at box 3, here is the list of possiblities:

r1c8: 1, 7, 9

r1c9: 1, 7, 9

r2c9: 2, 8

r3c7: 2, 7, 8, 9

r3c8: 7, 9

What is intresting in this box is that there are only two cells that can have the numbers 2 and 8, and that is r2c9 and r3c7.

This means that both the 7 and the 9 in r3c7 are impossible (it is has to be a 2 or an 8).

Now let's look at column 7:

r3c7: 2, 7, 8, 9 ==> We just said that 7 and 9 aren't possible, so it really is 2, 8

r4c7: 5, 8, 9

r5c7: 5, 7, 8, 9

r6c7: 5, 7, 9

r7c7: 2, 5, 9

What is intresting here is the number 7, it can only go in r5c7 or r6c7, both of these cells are part of box 6. This allows us to exclude the 7 from all other cells in box 6.

Box 6:

r3c7: 5, 8, 9

r4c7: 5, 7, 8, 9

r5c8: 1, 6, 7, 9 ==> 7 is not possible here, the 7 of box 6 has to be in column 7, so it really is 1, 6, 9

r5c9: 1, 5, 7, 8, 9 ==> no 7 either ==> 1, 5, 8, 9

r6c7: 5, 7, 9

r6c8: 6, 7, 9 ==> the 7 here is impossible(!) ==> 6, 9

Now when you look at row 6 you will see that there are only two cells that can have the number 6 and 9. This allows you to exclude the 9 from r6c1 leaving only one place for the 9 in box 4.

Did this help?