Prove: Let E be a finite extension of a field F. Let F₁ and F₂ be subfields of E containing F. Then F₁+F₂ is not a subfield of E, except in the cases when F₁ is subset of F₂ or F₂ is subset of F₁.
Prove: Let E be a finite extension of a field F. Let F₁ and F₂ be subfields of E containing F. Then F₁+F₂ is not a subfield of E, except in the cases when F₁ is subset of F₂ or F₂ is subset of F₁.
Suppose . Now the question is: where does belong to?