f(x)=sin^2x+mcosx-1
=1-cos^2x+mcosx-1
=-cos^2x+mcosx
=-(cosx-m/2)^2+m^2/4
令t=cosx,则f(t)=-(t-m/2)^2+m^2/4=-t^2+mt
∵t=cosx∈[-1,1]
∴当t=m/2∈[-1,1],即m∈[-2,2]时,最大值g(m)=m^2/4;
当t=m/2∈(-∞,-1],即m∈(-∞,-2]时,最大值g(m)=f(-1)=-1-m;
当t=m/2∈[1,+∞),即m∈[2,+∞)时,最大值g(m)=f(1)=-1+m;