Sudoku puzzle - where to go next?

Above is a link to a half-solved sudoku puzzle. I tried to fully solve it but just cant figure out what to do now.

Source: Sudoku-Branium studios It is an app on google playstore.

• A 9 in the top left corner of the bottom right square.
– Rick
Mar 2, 2018 at 14:02
• @RickvanOsta; is that because a 7 would force the 7 in block 2,3?
– JMP
Mar 2, 2018 at 14:26
• @JonMarkPerry Welp, I had a theory for it which had to do with the 5 in the top row. I seem to have forgotten why exactly that had to be a 9, but I tried solving the sudoku with the 9 in that place and everything seems to fit, which leaves me to believe that it's the correct position either way, haha.
– Rick
Mar 2, 2018 at 15:00
• Related: techniques for solving a Sudoku. Mar 2, 2018 at 17:12

2 Answers

In column 9, the two squares at locations (6,9) and (8,9) can both contain only a 5 or a 7. Any other digit at either location would cause an immediate clash with a similar digit in its row or column.
It follows that one of them is 5 and the other is 7, and therefore that 5 and 7 cannot occur elsewhere in column 9. This technique is called a "naked pair". Location (3,9) can therefore only be an 8.
After that, everything is straightforward.

• How do you reach the conclusion that (8,9) must have those numbers only?
– Sid
Mar 2, 2018 at 14:18
• @Sid Column 9 and row 8 together already contain all the other numbers, so only a 5 or 7 there would not cause a direct conflict. Mar 2, 2018 at 14:23

The key is in the row 7 and column 4.

• I think the question was about the 'next step' not the complete answer. Watching this image kinda spoils the puzzle for the OP. Mar 2, 2018 at 15:19
• @PL457, I don't think the OP asked for the next step. And I also don't see him complaining about my answer spoiling the puzzle for him, you might have the ability of mind reading :) Mar 2, 2018 at 16:13
• I dont want to complain or anything, but PL457 is right. Mar 2, 2018 at 17:15