Fill in the grid with each row and column having all of the numbers 1 to 5.
The "spots" are the results of diagonal sums.
If a spot shows a + and a 7, then the top left and bottom right values will add together to 7, and also the top right and lower left values will also equal 7.

You can Print out a Sheet of Mathrax, or alternatively Play it in your browser.

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What I Didn't Do

I was slightly tempted to increase the size of the grid, but I haven't made any suitable grids other than the 5's. (The grid is taken from Wordoku!)
I might have to create a pre-generator and build a bunch of suitable grids at different sizes, for future reuse.