How can I solve this Sudoku without guessing?

I am fairly new to Sudoku and have hit a brick wall with this one. My colleague said it can only be solved with a guess and seeing how far you go. However, I agree with many on this site and you should not need to guess…it is a mathematical problem so no need to guess. I believe I have gone as far as I can, but welcome someone telling how to get one more number without guessing. Thanks.

(The blue cells were pre-completed.)

• You forgot to clear the pencil mark 2s in the top right box. May 10, 2019 at 19:27
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– Rubio
May 12, 2019 at 8:11

1 Answer

Let's call the columns numerically from left to right 1...9, and the rows alphabetically from top to bottom A...I. We'll refer to a cell using its row letter, followed by the column number.

We are going to use

the XY Wing technique

Let's consider the cells

B1, C2, D1

They conform to that pattern, with

- B1 being the middle cell
- C2 and D1 being the wings

We are sure that

either C2 or D1 must be a 9

which means

D2 can't be a 9

Now considering the cells

D2 and F3, they must contain 1 and 2.
They are a Naked Pair
⇒ D1 must be a 9

And then you'll be able to remove some candidates from

the G row.

You can take it from here or use this website to get hints or solve the puzzle either partially or completely.

• Thanks ielyamani . I will give this a go. I find the XWing technique a bit confusing, but your explanation is great. Thanks again.
– SSD
May 10, 2019 at 7:45