أَنْتَ مُجْتَهِدٌ — أَنْتِ مُجْتَهِدَةٌ
Translation: You are diligent (masculine) — you are diligent (feminine).
Both forms address an identified listener and remain pronouns, hence definite.
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Pronouns of Address
Conclusion: Definite and Indefinite Nouns
أَنْتَ مُجْتَهِدٌ — أَنْتِ مُجْتَهِدَةٌ
Translation: You are diligent (masculine) — you are diligent (feminine).
Both forms address an identified listener and remain pronouns, hence definite.