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Mathematics
Problem
A, B, C and D are four points in space. Using vector methods, prove that
A
C
2
+
B
D
2
+
A
D
2
+
B
C
2
≥
A
B
2
+
C
D
2
AC^2 + BD^2 + AD^2 + BC^2 \ge AB^2 + CD^2
A
C
2
+
B
D
2
+
A
D
2
+
B
C
2
≥
A
B
2
+
C
D
2
what is the implication of the sign of equality.
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