# [SOLVED]mortifiedpenguin1's Question from Math Help Forum

#### Sudharaka

##### Well-known member
MHB Math Helper
Title: How do you do this problem?

Please offer step by step solutions There are three vectors. They are v= (3,-1,2), b= (4,2,-5) and n= (1,3,-7). Please prove that they form a closed triangle. What type of triangle is it?

Thanks!!!
Hi mortifiedpenguin1, I presume what you meant by a "closed triangle" is in fact to show that the given three points are non-collinear; so that a triangle is uniquely determined.

You can show that the points are not collinear by showing that the two vectors $$\overrightarrow{vb}\mbox{ and }\overrightarrow{vn}$$ are not parallel. That is their cross product, $$\overrightarrow{vb}\times\overrightarrow {vn}\neq\underline{0}$$

$\overrightarrow{vb}=\mathbf{b}-\mathbf{v}=(4,2,-5)-(3,-1,2)=(1,3,-7)$

$\overrightarrow {vn}=\mathbf{n}-\mathbf{v}=(1,3,-7)-(3,-1,2)=(-2,4,-9)$

$\Rightarrow\overrightarrow {vb}\times\overrightarrow {vn}=\begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k}\\1 & 3 & -7\\-2 & 4 & -9 \end{vmatrix}=(1,23,10)\neq\underline{0}$

Therefore the three points are not collinear, and hence they determine a unique triangle.

We shall also find the vector, $$\overrightarrow {bn}$$

$\overrightarrow {bn}=\mathbf{n}-\mathbf{b}=(1,3,-7)-(4,2,-5)=(-3,1,-2)$

Consider the length of the vectors, $$\overrightarrow{vb},\, \overrightarrow {vn}\mbox{ and }\overrightarrow {bn}$$.

$|\overrightarrow{vb}|=\sqrt{1^2+3^2+7^2}=\sqrt{59}$

$|\overrightarrow {vn}|=\sqrt{2^2+4^2+9^2}=\sqrt{101}$

$|\overrightarrow {bn}|=\sqrt{3^2+1^2+2^2}=\sqrt{14}$

Let $$\theta$$ be the angle between the sides, $$\mbox{vb}$$ and $$\mbox{bn}$$. By the Cosine rule,

$|\overrightarrow {vn}|^2=|\overrightarrow{vb}|^2+|\overrightarrow {bn}|^2-2|\overrightarrow{vb}||\overrightarrow {bn}|\cos\theta$

$\Rightarrow\cos\theta=-\frac{(101-59-14)}{2\sqrt{14}\sqrt{59}}\approx -0.4871$

$\Rightarrow\theta\approx 119.1^{0}$

Therefore this is an Obtuse triangle.

Kind Regards,
Sudharaka.

• Ackbach

#### biffboy

##### New member
b-v-n=0 Hence we have a closed triangle

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