Chuyển các câu sau sang dạng bị động: How have you solved this difficult math question

Chuyển các câu sau sang dạng bị động
1 How have you solved this difficult math question 
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2 she hasn’t prepared for dinner since 6.30pm
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3 The company has hired new workers 
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4 Have the thieves stolen the most valuable painting in the national museum?
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5. Have you finished the above sentences ?
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6. have you sent the Christmas cards to your family?