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Tuyển chọn các dạng toán hay lạ khó - Chu Văn Biên (Phần I-IV)

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Khi đó: U U 2U<br />

240(V) Chọn B.<br />

AM<br />

MB max<br />

Câu 22.<br />

Áp dụng định lí hàm số sin cho<br />

AB<br />

AM <br />

MB<br />

AM MB<br />

AMB :<br />

sin sin sin sin <br />

sin <br />

2 <br />

U U U U<br />

AM MB AM MB<br />

<br />

<br />

<br />

2sin cos 2sin cos<br />

2 2 4 2 2<br />

<br />

sin <br />

4 2 <br />

U U <br />

2U<br />

cos max .<br />

AM MB AB<br />

2<br />

sin <br />

2 <br />

<br />

sin <br />

Khi đó:<br />

4 2<br />

U U <br />

2U<br />

330 Chọn B.<br />

AM MB max AB<br />

<br />

sin <br />

2 <br />

Câu 23.<br />

<br />

<br />

Ta đã biết: U U<br />

max khi ΔAMB cân tại M, suy ra:<br />

cd<br />

C<br />

2 2<br />

R R<br />

ZC ZRL R ZL cosRL<br />

0,8.<br />

R Z ZC<br />

2 2<br />

L

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