问题 选择题

若实数x,y满足9x+9y=3x+1+3y+1,则u=3x+3y的取值范围是(  )

A.3<u≤6

B.0<u≤3

C.0<u≤6

D.u≥6

答案

∵9x+9y=3x+1+3y+1,9x+9y=(3x+3y2-2•3x•3y=u2-2•3x•3y,3x+1+3y+1 =3u,

∴u2-2•3x•3y=3u,

∴3x•3y=

u2-3u
2

又 0<3x•3y(

u
2
)2

∴0<

u2-3u
2
(
u
2
)
2
,即 
u(u-3)>0
u(u-6)≤0
,解得3<u≤6.

故选:A.

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