Question

Giúp mik bài 3 nhen -.-"

Answer 1

a) Ta có: AD\(\perp\)AB(gt)

BC\(\perp\)AB(gt)

Do đó: AD//BC(Định lí 1 từ vuông góc tới song song)

b) Ta có: AD//BC(cmt)

nên \(\widehat{D}+\widehat{C}=180^0\)(hai góc trong cùng phía)

\(\Leftrightarrow4\cdot\widehat{C}=180^0\)

hay \(\widehat{C}=45^0\)

\(\Leftrightarrow\widehat{D}=3\cdot45^0=135^0\)

c) Ta có: \(\widehat{D}+\widehat{C}=180^0\)(cmt)

mà \(\widehat{D}-\widehat{C}=30^0\)

nên \(2\cdot\widehat{D}=210^0\)

hay \(\widehat{D}=105^0\)

\(\Leftrightarrow\widehat{C}=105^0-30^0=75^0\)