1. Vì sao a // b.

   Hướng dẫn:

1. Dựa vào tính chất quan hệ giữa tính vuông góc với tính song song.

2. Tính số đo góc C dựa vào tính chất hai góc trong cùng phía.

Bài giải:

1. a // b vì a và b cùng vuông góc với đường thẳng \(AB.\)

2. Ta có: a // b (chứng minh trên)

Suy ra \(\widehat{C} + \widehat{D} = 180^o\) (hai góc trong cùng phía)

\(\Rightarrow \widehat{C} = 180^o - \widehat{D} = 180^o - 120^o = 60^o\)

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Tóm tắt bài

Đề bài

Xem hình 31:

1. Vì sao a // b?

2. Tính số đo góc C.

Hướng dẫn giải

+ Áp dụng quan hệ giữa tính vuông góc và tính song song của ba đường thẳng.

+ Áp dụng tính chất của hai đường thẳng song song.

Lời giải chi tiết

1. a // b vì a và b cùng vuông góc với đường thẳng AB

2. Ta có:

\(\widehat C + \widehat D = {180^0}\)

(Vì hai góc trong cùng phía)

Nên \(\widehat C = {180^0} - \widehat D =180^o-120^o= {60^0}\)

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• 
  
  Cho góc xOy nhọn, có Ot là tia phân giác. Lấy điểm A trên tia Ox . Lấy điểm B trên Oy sao cho OA=OB. Vẽ đoạn thẳng AB cắt Ot tại M
• Chứng minh tam giác AOM=tam giác BOM
• Chứng minh AM=BM
• Lấy điểm H trên tia Ot. Qua H vẽ đường thẳng song song với AB, đường thẳng này cắt Ox tại C, cắt Oy tai D. Chứng minh OHvuông góc với CD

cho a song song với b, vẽ đường thẳng a cắt A tại và cắt b tại B. chứng tỏ hai tia phân giác của hai góc trong cùng phía vuông góc với nhau.