# How to take a dot product of vectors “in equations”

I know the terminology is very weird and probably misled many people form what I’m actually asking, but I don’t know any better way to describe it...

Why is the dot product of $$\ b_1$$ and $$\ b_2$$ equal to equation below? How did they even get those values?

Can someone guide me through the steps to get the answer?

$$\vec b_1 = \vec c_1 \cos \theta + \vec c_2 \sin \theta$$

$$\vec b_2 = \vec c_2 \cos \theta - \vec c_1 \sin \theta$$

$$\vec b_1 \cdot \vec b_2 = -(\vec c_1^2 - \vec c_2^2)\sin \theta \cos \theta + \vec c_1 \cdot \vec c_2(\cos^2 \theta - \sin^2 \theta) = 0$$

the equation in an image

[edit 1] wrote out the equation in MathJax

[edit 2] forgot to mention that $$b_1$$ and $$b_2$$ are assumed to be perpendicular

• Dot product is distributive over addition, so you can just expand the product. Dec 27, 2019 at 1:36
• Please make your question visible. You cannot "convert" that link into mathjax but you can type it in mathjax. Dec 27, 2019 at 1:36
• MathJax quick reference
– amd
Dec 27, 2019 at 1:45
• @eyeballforg I still don't understand what the simplified version of the dot product would be. I don't see any components of the vectors, can you please point them out? Dec 27, 2019 at 2:03
• @Hasith when eyeballfrog says “expand the product,” he doesn’t mean that you should write the product out in terms of its components Dec 27, 2019 at 2:13

Given $$\vec b_1$$ and $$\vec b_2$$ are perpendicular and their dot product is 0, we have: $$\vec b_1\cdot\vec b_2=(\vec c_1\cos\theta+\vec c_2\sin\theta)\cdot(\vec c_2\cos\theta -\vec c_1\sin\theta)=0$$ $$\Rightarrow \vec c_1\cdot\vec c_2 \cos^2\theta - |\vec c_1|^2 \sin\theta\cos\theta +|\vec c_2|^2\sin\theta\cos\theta-\vec c_1\cdot\vec c_2 \sin^2\theta=0$$ $$\Rightarrow \vec c_1\cdot\vec c_2(\cos^2\theta-\sin^2\theta)-(|\vec c_1|^2-|\vec c_2|^2)\sin\theta\cos\theta=0$$ Notice that $$\vec c_1 \cdot \vec c_2$$ is just a scalar quantity