Hướng dẫn giải Câu 1 (Trang 94 SGK Toán Hình học 12)

Trong các mệnh đề sau, mệnh đề nào sai? (A) <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="|" close="|"><mover><mi>a</mi><mo>→</mo></mover></mfenced><mo>=</mo><msqrt><mn>2</mn></msqrt></math>;                  (B) <math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced open="|" close="|"><mover><mi>c</mi><mo>→</mo></mover></mfenced><mo>=</mo><msqrt><mn>3</mn></msqrt></math>;                (C) <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></math>;                        (D) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>b</mi><mo>→</mo></mover><mo>⊥</mo><mover><mi>c</mi><mo>→</mo></mover></math>. Giải Trả lời: Ta có:       <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>b</mi><mo>→</mo></mover><mo>·</mo><mover><mi>c</mi><mo>→</mo></mover><mo>=</mo><mo> </mo><mn>1</mn><mo>+</mo><mn>1</mn><mo>+</mo><mn>0</mn><mo>=</mo><mn>2</mn><mo> </mo><mo>≠</mo><mn>0</mn></math>                      <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>⇒</mo></math>  <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>b</mi><mo>→</mo></mover></math> không vuông góc với <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>c</mi><mo>→</mo></mover></math>. Chọn (D).