∵a=7，b=3，c=5，且a为最大边，∴最大角为A，cosA= b2+c2?a2 2bc = 9+25?49 30 =- 1 2 ，∴A=120°；由正弦定理 a sinA = c sinC ，得sinC= csinA a = 5× 3 2 7 = 5 3 14 ．
