一道高中不等式证明题已知正数x,y,z满足x+y+z=1求证:x^2/(y+2z)+y^2/(z+2x)+z^2/(x+
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一道高中不等式证明题
已知正数x,y,z满足x+y+z=1
求证:x^2/(y+2z)+y^2/(z+2x)+z^2/(x+2y)>=1/3
已知正数x,y,z满足x+y+z=1
求证:x^2/(y+2z)+y^2/(z+2x)+z^2/(x+2y)>=1/3
由柯西不等式:
[(y+2z)+(z+2x)+(x+2y)][x^2/(y+2z)+y^2/(z+2x)+z^2/(x+2y)]>=(x+y+z)^2=1
且有(y+2z)+(z+2x)+(x+2y)=3(x+y+z)=3
所以x^2/(y+2z)+y^2/(z+2x)+z^2/(x+2y)>=1/3
证毕.
注:本题为2009年浙江省高考数学自选模块不等式选讲第一题.
[(y+2z)+(z+2x)+(x+2y)][x^2/(y+2z)+y^2/(z+2x)+z^2/(x+2y)]>=(x+y+z)^2=1
且有(y+2z)+(z+2x)+(x+2y)=3(x+y+z)=3
所以x^2/(y+2z)+y^2/(z+2x)+z^2/(x+2y)>=1/3
证毕.
注:本题为2009年浙江省高考数学自选模块不等式选讲第一题.
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