Hướng dẫn Giải Bài 20 (trang 54, SGK Toán 9, Tập 1)

Hướng dẫn giải Bài 20 (Trang 54 SGK Toán 9, Tập 1)

Bài 20 (Trang 54 SGK Toán 9, Tập 1): Hãy chỉ ra ba cặp đường thẳng cắt nhau và các cặp đường thẳng song song với nhau trong số các đường thẳng sau: <math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">a</mi><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo>+</mo><mn>2</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi mathvariant="normal">b</mi><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo>=</mo><mi mathvariant="normal">x</mi><mo>+</mo><mn>2</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi mathvariant="normal">c</mi><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo>=</mo><mn>0</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo>-</mo><mn>3</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi mathvariant="normal">d</mi><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo>=</mo><mi mathvariant="normal">x</mi><mo>-</mo><mn>3</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi mathvariant="normal">e</mi><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo>-</mo><mn>1</mn><mo>;</mo><mspace linebreak="newline"/><mspace linebreak="newline"/><mi mathvariant="normal">g</mi><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo>=</mo><mn>0</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo>+</mo><mn>3</mn><mo>.</mo></math>     Hướng dẫn giải: Hai đường thẳng cắt nhau là hai đường thẳng có <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>≠</mo><mi>a</mi><mo>'</mo></math>. Ta có ba cặp đường thẳng cắt nhau sau: <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>1</mn><mo>)</mo><mo> </mo><mi>y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>1</mn><mo>,</mo><mn>5</mn><mi>x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>2</mn><mo> </mo><mi>v</mi><mi>à</mi><mo> </mo><mi>y</mi><mo> </mo><mo>=</mo><mo> </mo><mi>x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>2</mn></math> <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>1</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>2</mn><mo> </mo><mo> </mo><mi>và</mi><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>0</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>-</mo><mo> </mo><mn>3</mn></math> <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>3</mn><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>1</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>2</mn><mo> </mo><mi>và</mi><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mi mathvariant="normal">x</mi><mo> </mo><mo>-</mo><mo> </mo><mn>3</mn></math>. Hai đường thẳng song song là hai đường thẳng có <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>a</mi><mo>=</mo><mi>a</mi><mo>'</mo></math> và <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>b</mi><mo>≠</mo><mi>b</mi><mo>'</mo></math>. Ta có ba cặp đường thẳng song song sau: <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>1</mn><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>1</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>2</mn><mo> </mo><mi>và</mi><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>1</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>-</mo><mo> </mo><mn>1</mn></math> <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>2</mn><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mi mathvariant="normal">x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>2</mn><mo> </mo><mi>và</mi><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mi mathvariant="normal">x</mi><mo> </mo><mo>-</mo><mo> </mo><mn>3</mn></math> và <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi><mo>=</mo><mi>x</mi><mo>-</mo><mn>3</mn></math> <math xmlns="http://www.w3.org/1998/Math/MathML"><mn>3</mn><mo>)</mo><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>0</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>+</mo><mo> </mo><mn>3</mn><mo> </mo><mi>và</mi><mo> </mo><mi mathvariant="normal">y</mi><mo> </mo><mo>=</mo><mo> </mo><mn>0</mn><mo>,</mo><mn>5</mn><mi mathvariant="normal">x</mi><mo> </mo><mo>-</mo><mo> </mo><mn>3</mn><mo>.</mo></math>