求y=(3sinx+1)/(sinx+2)的值域

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求y=(3sinx+1)/(sinx+2)的值域

求y=(3sinx+1)/(sinx+2)的值域
求y=(3sinx+1)/(sinx+2)的值域

求y=(3sinx+1)/(sinx+2)的值域
令m=sinx,则y=3m^2+7x+2=3(x+1)^2+(x-1)
再令t=x+1,t定义域为[0,2],y=3t^2+t-2.
所以最低点取t=-1/3,但不在定义域内.
y在[0,2]上单调递增,所以值域为[-2,12]

令a=sinx
则-1<=a<=1
y=(3a+1)/(a+2)=[3(a+2)-5]/(a+2)
=3(a+2)/(a+2)-5/(a+2)
=3-5/(a+2)
-1<=a<=1
1<=a+2<=3
1/3<=1/(a+2)<=1
-1<=-1/(a+2)<=-1/3
-5<=-5/(a+2)<=-5/3
3-5<=3-5/(a+2)<=3-5/3
-2<=3-5/(a+2)<=4/3
所以值域[-2,4/3]