函数y=sinx/根号(5+4cosx) [0<=x<=2π]的值域(并清竖用绝大不等式求解)y^2=sin^2(x)/(5+4cosx),令t=(5+4cosx)∈[1,9] sin^2=1-[(t-5)/4]^216y^2=10-(t+9/t))∈[0,4] [用基正兄本不等式]y∈[-1/2,1/2]