假设两个长方形的长分别是a1和a2,宽分别是b1和b2 则有a1*b1 = a2*b2 而且有a1:a2 =3 :7 则a1 = 3/7 *a2 代入上式有 3/7 *a2 *b1 =a2 *b2 两边除以a2 得 3/7 *b1 =b2 则b1:b2 =7:3
