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How the left-hand side of the equation be transformed into the right-hand side?

Posted 1 day ago

This equation holds true:

x (E^x + 1) Log[x] - x E^x + x^2 + x + 
  1 == (x + Log[x]) (E^x + 1) x - (x E^x - 1) (x + 1)

How the left-hand side of the equation be transformed into the right-hand side?

x (E^x + 1) Log[x] - x E^x + x^2 + x + 1 // Expand // 
 Collect[#, {(E^x + 1) x, (x + 1)}] &

The method above doesn’t work.

POSTED BY: Bill Blair

Assuming you know the desired output form:

x (E^x + 1) Log[x] - x E^x + x^2 + x + 1 /.
  Log[x] -> u - x //
 Collect[#, u, Factor] /. u -> Log[x] + x &
(*  -((1 + x) (-1 + E^x x)) + (1 + E^x) x (x + Log[x])  *)

If I didn't know the desired form, I'd probably investigate by hand, unless I knew from the original problem why one of the forms x + Log[x], x E^x - 1, E^x + 1, etc. was relevant to the problem. Then I would know to put things in terms of it, the way I put the above in terms of u = x + Log[x]. We were fortunate here that Log[x] appears in only one term in the desired form. To replicate the approach with u = x + 1, you'd have to figure out which of the x to substitute — and then make only that substitution. Not easy.

POSTED BY: Michael Rogers
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