怎么化简这两个公式80-30sin（π/12t)=60-cos（π/12t)化简的到30sin(π/12t)-40cos(π/12t)-20=0 T有几个值,我用画图计算机显示两个函数有两个交点.但是解不出来

令π/12t = a (a≠0),则t=π/12a80-30sin a =60- cos a30sin a - 20 =cos a两边平方,得 900sin ²a -1200sin a + 400 =cos ²a =1- sin ²a即901sin ²a - 1200sin a + 399 =0； ∴(sin a - 600/901...