已知实数x,y,满足3x+5y>3-y+5 -x,则下列式子成立的是( ) A.x+y<0 B.x+y>0 C.x-y<0 D.x-y>0
问题描述:
已知实数x,y,满足3x+5y>3-y+5 -x,则下列式子成立的是( )
A. x+y<0
B. x+y>0
C. x-y<0
D. x-y>0
答
考察函数f(x)=3x-5-x,它在R上是单调增函数,
由题意得:3x-5-x>3-y-5y,
f(x)=3x-5-x,f(-y)=3-y-5y,
∴f(x)>f(-y),从而得到:x>-y
即x+y>0.
故选B.