質問: 2cos2x+cosx-1=0 2cos2x+cosx-1=0
回答: Cosx=t, 2t^2+t-1=0, t1=-1, t2=1/2. cosx=-1, x=n+2nk, cosx=1/2, x=+-arccos(1/2)+2nl, lєZ, x=+-n/3+2nl, lєZ.