Question

# If the vectors PQ=-3i+4j+4k and PR=5i-2j+4k are the sides of a $\vartriangle PQR$, then the length of the median through P isA.$\sqrt {14}$B.$\sqrt {15}$C.$\sqrt {17}$D.$\sqrt {18}$E.$\sqrt {19}$

$\Rightarrow \overrightarrow {PQ} + \overrightarrow {PR} = 2\overrightarrow {PD}$
PD$= \frac{1}{2}\left| {PQ + PR} \right|$
PD$= \frac{1}{2}\left| {2i + 2j + 8k} \right|$
PD$= \frac{1}{2}\sqrt {4 + 4 + 64} = \frac{1}{2}\sqrt {72} = \sqrt {18}$
$\therefore$ The length of the median through P = $\sqrt {18}$
Note: Median of a triangle is a line segment joining a vertex to the midpoint of the opposite side thus bisecting that side. In a triangle while using vectors the sum of two sides is equal to one particular side. $\Rightarrow \overrightarrow {PQ} + \overrightarrow {PR} = \overrightarrow {QR}$, If D is the midpoint of the side QR, then QR = 2QD = 2RD. After drawing a line from P to D, the $\vartriangle PQR$ divided into two triangles both have PD side as common. Let’s take $\vartriangle PQD$$(or\vartriangle PRD)$ to find the length of PD.$\Rightarrow PQ + QD = PD$, We know that QD is half of QR. Then we can find PD i.e., length of median through P.