Ta co: ab/bc=b/c => a/c=b/c  => a=b

Xet dang thuc ac=b² ta co: ac=b² 

thay a=b vao dang thuc tren ta duoc:  b.c=b² hay b.c=b.b => c=b

Ma b=a nen suy ra c=b=a

Vi a=b ; c=b nen suy ra ac=b² (dpcm)

1.

Traveling/ train/ be/ slower/ but/ interesting/ traveling/ plane.

Traveling by train is slower but more interesting than traveling by plane

You/ know/ who/ invent/ telephone?

Do you know who invented the telephone?

At recess/ yesterday/ Trang/ drink/ water/ talk/ friends

At recess in yesterday,Trang drank water and talked with friends

2. Make question

I started teaching English in 1996. When did you start teaching English.?

My family watches T.V every night. But now we are listening to pop music

Mrs Mai makes all her own clothes. At the moment she is making a dress for her daughter.