Sudoku Players' Forum (clone)

I found a Killer Sudoku on the krazydad site. I downloaded a PDF and the specific puzzle is Killer Sudoku #4. I have already spent more than a fair amount of time on this puzzle, but am stuck. I have checked each row, column, and square. I looked for pattersn of matchign numbers. I have checked the formulas and have also looked for hidden doubles and triples. I have no idea what to try next – could you please offer some guidance.

This http://pairofdocs.net/wp-content/uploads/2023/05/Stuck_KillerSudoku.jpg shows as far as I have gotten.

Each box has 1 to 9. Sum is 45.
Box 7 has 3 areas: 20+8+16, sum 44.
What can be in r7c1?

That hadn't occurred to me. Thanks!

I recommend Andrew Stuart's site for the 'classic' Sudoku and for many variants of Sudoku. It is quite easy to enter your Killer and then to see some solution paths:

www.sudokuwiki.org/killersudoku.htm