根据已知角求未知角

解析：
根据正弦定理：(AB/BC)×(BC/CD)×(CD/AD)×(AD/AB)＝1 ☆★☆ (sin(270/7)/sin(480/7))×(sin(390/7)/sin(270/7))×(sin((540/7)－x)/sin(330/7))×(sin(240/7)/sinx)＝1，
(sin((540/7)－x)/sinx)＝(sin(480/7)sin(330/7)/sin(240/7)sin(390/7))＝(2sin(240/7)cos(240/7)sin(330/7)/sin(240/7)sin(390/7))＝(cos(240/7)sin(330/7)/sin30sin(390/7))
＝(sin(90－(240/7))sin(330/7)/sin30sin(390/7))＝(sin(390/7)sin(330/7)/sin30sin(390/7))＝(sin(330/7)/sin30)＝(sin((540/7)－30)/sin30)，x＝30。
注意：用到2倍角公式：sin(480/7)＝2sin(240/7)cos(240/7)，用到诱导公式：sin(90－(240/7))＝cos(240/7)。