MHB The exact value of cos(θ) in the form of p/q

  • Thread starter Thread starter karush
  • Start date Start date
  • Tags Tags
    Form Value
AI Thread Summary
The discussion focuses on determining the value of cos(θ) in the context of a triangle OAB, where the length OC is needed to calculate cos(θ) as OC/OA. The participants explore methods to find point C, suggesting converting vectors a and b into lines to find their intersection. They also reference the Law of Cosines and vector dot product calculations to derive cos(θ). Ultimately, they confirm that the length of OC is 11.2, leading to the conclusion that cos(θ) equals 56/65.
karush
Gold Member
MHB
Messages
3,240
Reaction score
5
View attachment 1401
calculations and boxed answers are mine

my question is with (iii) (provided the (i) and (ii) are correct.

from the diagram $cos \theta$ would be $\frac{OC}{OA}$ or $\frac{11.2}{13}$

but would need the the length of $OC$ to do so, what would be the best approach to get point $C$

we could convert $a$ and $b$ and to lines and find the intersection at $C$ hence $OC$.

but not sure how this could be done with just vectors
 
Mathematics news on Phys.org
Re: the exact value of cos theta in the form of p/q

karush said:
View attachment 1401
calculations and boxed answers are mine

my question is with (iii) (provided the (i) and (ii) are correct.

from the diagram $cos \theta$ would be $\frac{OC}{OA}$ or $\frac{11.2}{13}$

but would need the the length of $OC$ to do so, what would be the best approach to get point $C$

we could convert $a$ and $b$ and to lines and find the intersection at $C$ hence $OC$.

but not sure how this could be done with just vectors
Consider a triangle OAB. You can find the length of AB as you know the position vectors for A and B. Then use the Law of Cosines to find your angle.

-Dan
 
Re: the exact value of cos theta in the form of p/q

karush said:
View attachment 1401
calculations and boxed answers are mine

my question is with (iii) (provided the (i) and (ii) are correct.

from the diagram $cos \theta$ would be $\frac{OC}{OA}$ or $\frac{11.2}{13}$

but would need the the length of $OC$ to do so, what would be the best approach to get point $C$

we could convert $a$ and $b$ and to lines and find the intersection at $C$ hence $OC$.

but not sure how this could be done with just vectors

Use the fact that \displaystyle \begin{align*} \mathbf{a} \cdot \mathbf{b} = |\mathbf{a}||\mathbf{b}|\cos{(\theta)} \end{align*}

Also note that you have evaluate \displaystyle \begin{align*} \mathbf{a}\cdot \mathbf{b} \end{align*} incorrectly.

\displaystyle \begin{align*} \mathbf{a}\cdot \mathbf{b} &= 12 \cdot 6 + 5 \cdot 8 \\ &= 72 + 40 \\ &= 112 \end{align*}
 
Re: the exact value of cos theta in the form of p/q

$o^2=a^2+b^2-2ab\ cos\ \theta$

or $\displaystyle\frac{o^2 -a^2-b^2}{-2ab}=cos\ \theta$

plugging in $\displaystyle o=3\sqrt{3}\ a=10\ b=13$ we get $cos\ \theta = \frac{56}{65}$

well easier than finding $OB...$
 
Last edited:
Re: the exact value of cos theta in the form of p/q

Prove It said:
Use the fact that \displaystyle \begin{align*} \mathbf{a} \cdot \mathbf{b} = |\mathbf{a}||\mathbf{b}|\cos{(\theta)} \end{align*}

Also note that you have evaluate \displaystyle \begin{align*} \mathbf{a}\cdot \mathbf{b} \end{align*} incorrectly.

\displaystyle \begin{align*} \mathbf{a}\cdot \mathbf{b} &= 12 \cdot 6 + 5 \cdot 8 \\ &= 72 + 40 \\ &= 112 \end{align*}
$\displaystyle\frac{a\cdot b}{10\times 13} = \frac{112}{130}=\frac{56}{65}$
i reduced the fraction..
 
Re: the exact value of cos theta in the form of p/q

This may be something along the lines of what you were looking for:

Suppose we write the point $C$ as: $(6t,8t)$.

Clearly the length of $OC$ is $10t$.

The length of $AC$ is: $\sqrt{(8t - 5)^2 + (6t - 12)^2} = \sqrt{100t^2 - 224t + 169}$.

Because we have a right triangle, we know that:

$100t^2 + 100t^2 - 224t + 169 = 169$
$200t^2 - 224t = 0$
$t(25t - 28) = 0$

We can discount the solution $t = 0$, leaving us with $t = \frac{28}{25} = \frac{112}{100} = 1.12$, which establishes that the length of $OC$ is 11.2, as we surmised by the other method.
 
Thread 'Video on imaginary numbers and some queries'
Hi, I was watching the following video. I found some points confusing. Could you please help me to understand the gaps? Thanks, in advance! Question 1: Around 4:22, the video says the following. So for those mathematicians, negative numbers didn't exist. You could subtract, that is find the difference between two positive quantities, but you couldn't have a negative answer or negative coefficients. Mathematicians were so averse to negative numbers that there was no single quadratic...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Thread 'Unit Circle Double Angle Derivations'
Here I made a terrible mistake of assuming this to be an equilateral triangle and set 2sinx=1 => x=pi/6. Although this did derive the double angle formulas it also led into a terrible mess trying to find all the combinations of sides. I must have been tired and just assumed 6x=180 and 2sinx=1. By that time, I was so mindset that I nearly scolded a person for even saying 90-x. I wonder if this is a case of biased observation that seeks to dis credit me like Jesus of Nazareth since in reality...
Back
Top