设两个直角三角形ABC和A’B’C’,直角边AB/BC=2/3,A’B’/B'C’=2/3,
则AB/BC=A’B’/B'C’,AB/A'B'=BC/B'C',(更比)
而它们的夹角为90度,
故△rtABC∽rt△A'B'C.
证毕.