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问题: 救命~~~~~~~数学~~~

函数y= (1-sinacosa)/(1+sinacosa),当时0<=a<=π时, 最大值是___
怎么做?过程 ~~谢谢~~

解答:

y=(1-sinacosa)/(1+sinacosa)
=[2-(1+sinacosa)]/(1+sinacosa)
=[2/(1+sinacosa)]-1
=[4/(2+2sinacosa)]-1
=[4/(2+sin2a)]-1
sin2a=-1,既:2a=3π/2,a=3π/4时,ymax=3
2a=π/2,既:a=π/4时,ymin=1/3