Hướng dẫn giải Bài 9 (Trang 40 SGK Toán Đại số 8, Tập 2)

Cho tam giác ABC. Các khẳng định sau đúng hay sai: a) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>></mo><mn>180</mn><mo>°</mo></math>                           b) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo><</mo><mn>180</mn><mo>°</mo></math> c) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>≤</mo><mn>180</mn><mo>°</mo></math>                                 d) <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>≥</mo><mn>180</mn><mo>°</mo></math> Hướng dẫn: a) Vì <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>=</mo><mn>180</mn><mo>°</mo></math> nên khẳng định <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>≤</mo><mn>180</mn><mo>°</mo></math> đúng, do đó khẳng định <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>></mo><mn>180</mn><mo>°</mo></math> là sai. b) Do số đo của một góc của tam giác là một số dương nên <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>></mo><mn>0</mn><mo>°</mo></math>.          <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>=</mo><mn>180</mn><mo>°</mo><mo>,</mo><mo> </mo><mo> </mo><mo> </mo><mo> </mo><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>=</mo><mn>180</mn><mo>°</mo><mo>-</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo><</mo><mn>180</mn><mo>°</mo></math>    Khẳng định <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo><</mo><mn>180</mn><mo>°</mo></math> là đúng.  c) Do <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo><</mo><mn>180</mn><mo>°</mo></math> nên khẳng định <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">C</mi><mo>^</mo></mover><mo>≤</mo><mn>180</mn><mo>°</mo></math> đúng. d) Do khẳng định <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo><</mo><mn>180</mn><mo>°</mo></math> đúng, nên khẳng định ngược  <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi mathvariant="normal">A</mi><mo>^</mo></mover><mo>+</mo><mover><mi mathvariant="normal">B</mi><mo>^</mo></mover><mo>≥</mo><mn>180</mn><mo>°</mo></math> là sai.