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I'd like to solve the following two equations for P. But I'm getting "Solve was unable to determine the units of quantities that appear in the input.". Any suggestions on what could be the cause for this?

Clear["Global`*"] ;
Solve[{T^4 == ((1 - A) (1 - P) Subscript[R, s]^2 Subscript[T, 
    eff]^4)/(2 a^2), Subscript[\[CapitalDelta]F, SE]/F == (\!\(
\*SubsuperscriptBox[\(R\), \(p\), \(2\)]\ *\ 
\*SubscriptBox[\(T\), \(day\)]\))/(
   Subscript[R, s ]* Subscript[T, eff])}, {P, Subscript[T, 
  eff]}, Reals]
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1 Answer 1

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Avoid the use of subscripted variables, they cause more trouble than they're worth. Use Format to display variables in any manner desired.

Clear["Global`*"]

Format[Tday] = Subscript["T", "day"];
Format[Teff] = Subscript["T", "eff"];
Format[Rs] = Subscript["R", "s"];
Format[Rp] = Subscript["R", "p"];
Format[ΔFse] = Subscript["ΔF", "SE"];

eqns = {T^4 == ((1 - A) (1 - P) Rs^2 Teff^4)/(2 a^2),
  ΔFse/F == (Rp^2 * Tday)/(Rs* Teff)}

enter image description here

Solve will then work

sol = Solve[eqns, {P, Teff}, Reals] // Simplify

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Verifying the solutions,

eqns /. sol[[1]] // Simplify

(* {True, True} *)
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  • $\begingroup$ excellent answer, solves also the issue with the subscripts. thanks $\endgroup$
    – Nickpick
    Feb 13, 2021 at 12:06

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