# Algebra

## Polynomial Integer Root Theorem

https://www.youtube.com/watch?v=fgBBJnhbMvI There are a couple of related theorems that we interchangeably call the “integer root theorem.” One tells us how to find that integer roots of a polynomial with integer coefficients. The second tells us that all rational roots of a monic polynomial with integer coefficients are integers. We prove both results in this video.

## Polynomial Rational Root Theorem

https://www.youtube.com/watch?v=Huok57RGUkM The rational root theorem gives us a finite process for finding all rational roots of a polynomial with integer coefficients. We prove the correctness of the procedure and describe how to use it.

## Why is $\deg(0)=-\infty$?

https://www.youtube.com/watch?v=iwS6y1POULQ It is a standard convention that the degree of the zero polynomial is negative infinity. That is, $$deg(0)=-infty.$$ We explore the reasons for this strange definition, justify it, and look at some of its implications in this video.

## Square Root of a Complex Number

https://www.youtube.com/watch?v=d0avql3E6ik We show how to explicity compute the square roots of a complex number that is given in rectangular form. We perform the derivation in a motivated way, instead of simply mechanically verifying the formula.

https://www.youtube.com/watch?v=t9RuFHcyQRc&t=60s The ordinary way of proving the quadratic formula $$x=frac{-bpmsqrt{b^2-4ac}}{2a}$$ is through a process called “completing the square.” But there is one step at the end which requires casework that almost everyone glosses over. In this video, we show a little-known modificiation of completing the square that avoids the need for any casework.

## $n^{\text{th}}$ Roots of a Complex Number

https://www.youtube.com/watch?v=V7JK9yqP7Q0 How can we find all of the complex $n^{text{th}}$ roots of a complex number, if the complex number is given in polar or trigonometric form? We show how to achieve this goal in our video.

## De Moivre’s Formula

https://www.youtube.com/watch?v=k2gO530rwmU De Moivre’s formula allows us to conveniently compute the powers of complex numbers that are written in polar or trigonometric form. We prove de Moivre in this video, which states that $$(re^{itheta})^n = r^n e^{i(ntheta)}.$$

## Complex Triangle Inequality

https://www.youtube.com/watch?v=b5WdpOp4r8A Similar to the real number triangle inequality that uses the absolute value, there is a triangle inequality for complex numbers that uses the complex modulus $$|z+w|le |z|+|w|.$$ We show how to prove this inequality here.