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I went up in difficulty on my Sudoku app (Enjoy Sudoku app on Android). I keep getting mostly through them; however, I keep getting stuck towards the end of the puzzle. An example is shown below. Can I have an explanation of the technique I'm not using?

sudoku

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There are many websites with online sudoku solvers that show you which technique can be used next. For example this one. It came up with a Y-wing configuration here:

If cell R1C2 were a 9 then R3C3 would be a 3, and R5C2 would be a 6. This would leave R4C3 with no possible value. Therefore R1C2 is not a 9.
Or conversely - whichever value R4C3 takes, one of R5C2 and R3C3 is a 9, so R1C2 is not a 9 (and neither is R3C2).

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My apologies for not knowing the correct terms, but my first thought is that

R7C4 has to be a 3 in order for there to be a unique solution.
Otherwise there would be two solutions, with R7C4, R9C4, R7C8 and R9C8 being either 2 or 4.

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In the right bottom block a choice between 4-2 must be made. Because a sudoku will have only ONE solution, the 4 in the middle bottom block can not be combined with a 2. (otherwise it has two solutions, because the 4-2 can switch places)

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