1,y=cos2xcosπ/5-2sinxcosxsin6π/5=cos2xcosπ/5+sin2xsinπ/5=cos(2x-π/5) 递增区间为（kπ+π/10,kπ+3π/5）
2,化简得,(tana)^4-2tana-1=0,tana=..,解不出来了
3,将cos（a-b）写成cos(a+3π/4+π/4-b-π)=-cos(a+3π/4+π/4-b),再化开来就方便多了
4,由韦达定理,tana+tanb=5,tana*tanb=6,∴tana=2,tanb=3或者tana=3,tanb=2,再利用(sina)^2+(cosa)^2=1,解出sina,cosa的值,sinb,cosb同理
cos(a-b)=cosacosb+sinasinb =...