题目

在，已知，． （1）求与角的值； （2）若角，，的对边分别为，，，且，求，的值． 答案：解：（1）∵，∴， 又∵，∴， ∵，且， ∴． （2）由正弦定理得，∴， 另由，得             ， 解得或（舍去）， ∴，．E-mail, as well as telephones, _______an important part in our daily communication. A. play B．have played C．are playing D. is playing