# Thread: Vector dot product question- due tomorrow, please help!!

1. ## Vector dot product question- due tomorrow, please help!!

2 Consider a triangle ABC and let D be a point on the line between A and B. Use thedot product to prove that|−→AC|2|−−→BD| + |−−→BC|2|−−→AD| − |−−→CD|2|−→AB| = |−→AB||−−→AD||−−→BD|.Hint: You may find it helpful to write everything in terms ofx =−−→AD, y =−−→DC, z =−−→BDusing the identities−→AC =−−→AD +−−→DC and −−→BC =−−→BD +−−→DC.

2. ## Re: Vector dot product question- due tomorrow, please help!! Originally Posted by TheBoy 2 Consider a triangle ABC and let D be a point on the line between A and B. Use thedot product to prove that|−→AC|2|−−→BD| + |−−→BC|2|−−→AD| − |−−→CD|2|−→AB| = |−→AB||−−→AD||−−→BD|.Hint: You may find it helpful to write everything in terms ofx =−−→AD, y =−−→DC, z =−−→BDusing the identities−→AC =−−→AD +−−→DC and −−→BC =−−→BD +−−→DC.
Your notation is so p*** poor as to be unreadable. Here is a guess.
You will not get help until you confirm. IS THIS CORRECT?
$\displaystyle {\left\| {\overrightarrow {AC} } \right\|^2}\left\| {\overrightarrow {BD} } \right\| + {\left\| {\overrightarrow {BC} } \right\|^2}\left\| {\overrightarrow {AD} } \right\| - {\left\| {\overrightarrow {CD} } \right\|^2}\left\| {\overrightarrow {AB} } \right\| = \left\| {\overrightarrow {AB} } \right\|\left\| {\overrightarrow {AD} } \right\|\left\| {\overrightarrow {BD} } \right\|$