若1x+1y+1z=1x+y+z=1,则x,y,z中,正数的个数为( ) A.1个 B.2个 C.3个 D.都有可能
问题描述:
若
+1 x
+1 y
=1 z
=1,则x,y,z中,正数的个数为( )1 x+y+z
A. 1个
B. 2个
C. 3个
D. 都有可能
答
不妨设x≥y>z,因为1x+1y+1z=1x+y+z=1,所以不可能都是正数.∵若假设都是正数,则x<x+y+z,则1x>1x+y+z,同理1y>1x+y+z,1z>1x+y+z,则1x+1y+1z>1x+y+z,与1x+1y+1z=1x+y+z,矛盾.∴可以先确定z<0.又∵有x...