Harry Weston
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 Apr 7 awarded Nice Answer Jan 20 comment Is is possible to obtain exactly 16 black cells? Sorry, ran out of time in my last attempt, which started off wrong.. Here is a derivation of the formula for the number of black cells. My reasoning is this, if the size of the grid is X rows and X columns: change r rows gives rX black cells; changing c columns reverts cr cells to white (crossing the r rows), and c(X-r) cells to black (between the r rows). The total of black cells is thus rX - cr +c(X-r), which simplifies to X(r+c)-2cr as above Jan 20 comment Is is possible to obtain exactly 16 black cells? I have tried this on smaller grids and get a different formula for the number of black cells left. My reasoning is this: Dec 9 awarded Necromancer Apr 30 awarded Scholar Apr 30 accepted What is graph of graph of $y = (x-1)\sqrt{x}$ Apr 30 asked What is graph of graph of $y = (x-1)\sqrt{x}$ Aug 9 awarded Critic Aug 9 revised relationship of polar unit vectors to rectangular corrected the description of the resolution of the radius vector. Aug 9 answered relationship of polar unit vectors to rectangular May 13 awarded Yearling Mar 12 answered Too old to start math Sep 27 revised What are imaginary numbers? I added references to two reviews Sep 26 comment What are imaginary numbers? Thank you @Martin. I followed the link and am suitably chastened, but I did, and do, enjoy reading this book. I still think it might well be a nice introduction to complex numbers for someone who has not become acquainted with them already. Sep 26 awarded Supporter Sep 26 answered What are imaginary numbers? Jul 27 awarded Autobiographer Jul 27 awarded Editor Jul 27 awarded Student Jul 27 awarded Teacher