#2:

- Code: Select all
` *-----------*`

|..2|6..|...|

|73.|...|9..|

|...|...|421|

|---+---+---|

|...|..4|.1.|

|4.9|5.8|2.3|

|.1.|7..|...|

|---+---+---|

|326|...|...|

|..1|...|.46|

|...|..9|8..|

*-----------*

There is only one place in box 1 for a 1. Hint: It can't be in column 2 or 3 because those columns already have a 1.

Similarly, there is only one place for a 1 in box 5. There is only one place for a 1 in box 9.

Put a 2 in box 9 and a 4 in box 6. Look at putting a 6 in boxes 3 and 8.

Then you can complete row 5. It lacks a 6 and a 7. The 6 can only go one place.

Now you can complete the 6s by putting one in box 1 and box 5 and box 6.

Continue this process of finding "hidden" singles.

When I solved this, I ran out of the easy to spot hidden singles, and had to resort to "naked" singles, which, despite the name, are actually harder to see because you usually have to use pencilmarks to find them.

Here is the point at which I looked at each unsolved square and penciled in its possible candidates:

- Code: Select all
` *-----------*`

|1.2|6.5|387|

|73.|..1|965|

|6..|.73|421|

|---+---+---|

|...|..4|618|

|469|518|273|

|.1.|7.6|594|

|---+---+---|

|326|..7|159|

|..1|352|746|

|...|169|832|

*-----------*

*--------------------------------------------------*

| 1 49 2 | 6 49 5 | 3 8 7 |

| 7 3 48 | 248 248 1 | 9 6 5 |

| 6 589 58 | 89 7 3 | 4 2 1 |

|----------------+----------------+----------------|

| 25 57 357 | 29 239 4 | 6 1 8 |

| 4 6 9 | 5 1 8 | 2 7 3 |

| 28 1 38 | 7 23 6 | 5 9 4 |

|----------------+----------------+----------------|

| 3 2 6 | 48 48 7 | 1 5 9 |

| 89 89 1 | 3 5 2 | 7 4 6 |

| 5 457 457 | 1 6 9 | 8 3 2 |

*--------------------------------------------------*

When you use pencilmarks, the naked single 5 in r9c1 (row 9, column 1) jumps right out at you.

So make r9c1 a 5, and remove the 5s from r4c1, r9c2 and r9c3. Now there is a naked single 2 in r4c1.

Continue spotting naked singles and updating the candidate grid. The puzzle solves very easily.