余弦定理：a^=b^+c^-2bc cosA
cosA=-1/2=（9+25-a^2）/2x3x5 得到a=根号19
正弦定理：a/sinA=b/sinB=c/sinC
sinB=bsina/a=2分之3倍的根号57
sinC=csina/a=2分是3倍的根号95