已知长方形的长宽比为a：b=k,对角线为c.
a=bk
由勾股定理得,a^2+b^2=c^2
(bk)^2+b^2=c^2
b=√〔c^2/(k^2+1)〕,a=k√〔c^2/(k^2+1)〕.