Hướng dẫn giải Câu 2 (Trang 94 SGK Toán Hình học 12)

Trong các mệnh đề sau, mệnh đề nào đúng? (A) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>a</mi><mo>→</mo></mover><mo>·</mo><mover><mi>c</mi><mo>→</mo></mover><mo> </mo><mo>=</mo><mn>1</mn></math>;         (B) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>a</mi><mo>→</mo></mover></math>, <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>b</mi><mo>→</mo></mover></math> cùng phương;         (C) <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>cos</mi><mfenced><mrow><mover><mi>b</mi><mo>→</mo></mover><mo>,</mo><mo> </mo><mover><mi>c</mi><mo>→</mo></mover></mrow></mfenced><mo>=</mo><mfrac><mn>2</mn><msqrt><mn>6</mn></msqrt></mfrac><mspace linebreak="newline"/></math>;          (D) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>a</mi><mo>→</mo></mover><mo>+</mo><mover><mi>b</mi><mo>→</mo></mover><mo>+</mo><mover><mi>c</mi><mo>→</mo></mover><mo>=</mo><mover><mn>0</mn><mo>→</mo></mover></math>. Giải Trả lời: Chọn (C). <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>cos</mi><mfenced><mrow><mover><mi>b</mi><mo>→</mo></mover><mo>,</mo><mover><mi>c</mi><mo>→</mo></mover></mrow></mfenced><mo>=</mo><mfrac><mrow><mover><mrow><mi>b</mi><mo>.</mo></mrow><mo>→</mo></mover><mover><mi>c</mi><mo>→</mo></mover></mrow><mrow><mfenced open="|" close="|"><mover><mi>b</mi><mo>→</mo></mover></mfenced><mo>.</mo><mfenced open="|" close="|"><mover><mi>c</mi><mo>→</mo></mover></mfenced></mrow></mfrac><mo>=</mo><mfrac><mn>2</mn><mrow><msqrt><mn>2</mn></msqrt><mo>.</mo><msqrt><mn>3</mn></msqrt></mrow></mfrac><mo>=</mo><mfrac><mn>2</mn><msqrt><mn>6</mn></msqrt></mfrac><mo>.</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mspace linebreak="newline"/><mspace linebreak="newline"/></math>