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enter image description here Got this from daily challenge of sudoku app.

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    $\begingroup$ Your sudoku image is a bit misleading because in the lower right quadrant, you don't have any 3's marked there for some reason. So I'm not sure that your marks can be trusted at all. $\endgroup$ – JS1 Feb 9 at 17:54
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It's certainly not an easy one to tackle here to find a number. There are probably easier ways to chip away at the pencil marks (after filling in the missing $3$s as per JMP's comment), but...

Labeling:
* the cells as $(column, row)$ with $(1,1)$ in the top-left; and
* sets as $\{\cdots\}$

If $(7,1)$ were a $3$
then $(7,8)$ is a $9$
and so $(6,8)$ would be a $7$
leaving $(6,1)$ as an $8$ and $(6,2)$ as a $3$
as such $(5,3)$ would be a $1$
so one of $\{(7,3), (8,3), (9,3)\}$ would be a $3$
but If $(7,1)$ were a $3$ none of $\{(7,3), (8,3), (9,3)\}$ could be a $3$
Therefore $(7,1)$ cannot be a $3$ and must be a $9$

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  • $\begingroup$ Idk how you guys have so good eyes to spot such tricks. I try to use pencil markings and eventually I get lost, similar to what happened in this puzzle $\endgroup$ – Kashish Maheshwari Feb 9 at 18:47
  • $\begingroup$ This was a "guess and check" rather than spotting a pattern. $\endgroup$ – Jonathan Allan Feb 9 at 18:50
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r5c7 can't be a $3$, because then r1c7 and r3c7 are $6$ and $9$, which forces the remaining 2 cells in the column to be $5,7$, which forces a $6,8$ in the right column of the lower-right block, which means both r7c8 and r9c8 are $2$. Also the is only one option for the $3$ in r9.

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