1.确有cos(2*pi+A)=cos(A)；
2.cos(2*pi*f*t+x1(t))+cos(2*pi*f*t+x2(t))≠cos(2*pi*f*t+[x1(t)+x2(t)]) .
应该是：cos(2*pi*f*t+x1(t))+cos(2*pi*f*t+x2(t))＝2cos(2*pi*f*t+[x1(t)+x2(t)]/2)cos[(x1(t)-x2(t))/2].
3.当且仅当k＝1时有“cos(2*pi*f*t+k*x(t)) = kcos(2*pi*f*t+x(t)) ”.但这是已然失去了“公式意义”.