已知1/x+1/y+z=1/2,1/y+1/z+x=1/3,1/z+1/x+y=1/4,求2/x+3/y+4/z的值.

问题描述:

已知

1
x
+
1
y+z
1
2
1
y
+
1
z+x
1
3
1
z
+
1
x+y
1
4
,求
2
x
+
3
y
+
4
z
的值.

1
x
+
1
y+z
1
2

x+y+z
x(y+z)
=
1
2

∴x(y+z)=2(x+y+z),
∴x=
2(x+y+z)
y+z

即:
1
x
=
y+z
2(x+y+z)

同理:
1
y
=
z+x
3(x+y+z)
1
z
x+y
4(x+y+z)

2
x
+
3
y
+
4
z
=
2(y+z)
2(x+y+z)
+
3(z+x)
3(x+y+z)
+
4(x+y)
4(x+y+z)
=
y+z
x+y+z
+
x+z
x+y+z
+
x+y
x+y+z
=
2(x+y+z)
x+y+z
=2.