I've moved onto "expert" sudoku's in my app (Sudoku.com on IOS) and whilst I am getting by solving some, others I'm having a hard time with.
With this one, I used a hint so the AI could note all the fields up for me but it hasn't put a note for 1 in this box. Why? I can't see anything which would rule it out for "certain". I know there are 2 possible 1's on the same line in block 9 but it also has the possibility of being on the line above so isn't certain in my mind.
Can someone explain why the AI can be sure no 1 should be in this box?