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malaygoel · #2 July 2nd, 2009, 08:44 PM

Quote:

Originally Posted by chengbin

$\overrightarrow {PD} = \frac {1}{2}(\overrightarrow {b}+\overrightarrow {c})$

Why? How do they get that?

from triangle addition of vectors,

$\overrightarrow {PD} = (\overrightarrow {b}+\overrightarrow {BD})$

$\overrightarrow {PD} = (\overrightarrow {c}+\overrightarrow {CD})$

adding above two equations,

$2\overrightarrow {PD} = (\overrightarrow {b}+\overrightarrow {BD}+\overrightarrow {c}+\overrightarrow {CD})$

now,
$\overrightarrow {CD}+\overrightarrow {BD}=\overrightarrow {0}$
since D is midpoint....hence the equation follows

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