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	Các bình luận về: Đề bài: Trong không gian cho bốn điểm $A,B,C,D$$a.$ Chứng minh hệ thức$AB^2+CD^2-AD^2-BC^2=2\overrightarrow {AC}.\overrightarrow {DB}  $$b.$ Tính giá trị biểu thức$\overrightarrow {AB} .\overrightarrow {CD}+\overrightarrow {AC}.\overrightarrow {DB}   +\overrightarrow {AD} .\overrightarrow {BC} $$c.$ Chứng minh rằng nếu $AB\bot CD$ và $AC\bot DB$ thì $AD\bot BC$	</title>
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