若函数Y=1/cosx-2的最大值为M,最小值为N,则A M-3N=0 B M+3N=0 C N-3M=0 D N+3M=0

-1≤cosx≤1,====>-1≤-cosx≤1,===>1≤2-cosx≤3.====>1/3≤1/(2-cosx)≤1.====>-1≤1/(cosx-2)≤-1/3.====>-1≤y≤-1/3.====>M=ymax=-1/3.N=ymin=-1.====>3M=-1,N=-1,===>3M=N.====>N-3M=0.====>选C.