The have an account on Khan Academy, that's all they need |
do you mean z^4 - 1 = 0? That can be factored using alg1 and introduces imaginary numbers. z^4 + 1 is much harder to factor. |
Nope, I meant z^4+1=0. (Now that I think about it, it was actually z^4 + 4 = 0, which gives nice integer solutions.) Don't use the polar form, though. Set z=a+bi and see where this gets you. |
Complex numbers were first used to solve cubics. If they show up in the quadratic formula, then there is no real solution. However, in the cubic formula, you can get complex numbers in the formula while having a real solution. |
Setting z = a+bi will work, but it's a quite lengthy to work through the algebra (expanding by binomial theorem, forming two separate equations and solving each of the cases). Much simpler is to rewrite the expression as the square of a binomial by completing the square, i.e: z^4 + 4 = (z^2 + 2)^2 - 4z^2 Now apply difference of squares to turn the above into a product of two quadratics. The four roots are easily extracted with the quadratic formula, two for each of the quadratics. (Bonus for students who have studied polar/exponential form of complex numbers: Plot the roots of z^4 + 4 in the complex plane, and also plot the roots of z^4 - 4... what interesting observation can you make? Can you explain how they are related to each other and exactly why this is the case?) |
I remember covering this with DC in 4th or 5th grade shortly before quadratics. |
My kid is now finishing Honors Algebra 2 in 9th grade (all As this year) and we never did any outside work. She says it is her favorite class, so she does like math. |
You have to realize the division between algebra 1 and 2 is somewhat arbitrary, although it’s true that national curriculums like common core omit complex numbers in elementary algebra. On the other hand elementary algebra textbooks for college are better structured and start with numbers (including complex) and arithmetic, then move to equations and then systems. Most good students can handle complex numbers when doing quadratic equations and I thinks it’s even recommended to do so, since it’s a more complete treatment of the topic. Also, some people choose enrichment to go deeper, broader and different then the worksheets kids do at school. |
Can you name one such book? |
Stewart, College Algebra Openstax, College Algebra All college algebra books, I’m not aware of a single one that doesn’t use complex numbers in the treatment of quadratics. The division between algebra, geometry and pre calculus is also quite arbitrary, with many areas of overlap. Of course arithmetic, equations and systems belong in algebra, shapes, angles, lines belong to geometry, and vectors and matrices traditionally taught in precalculus. But functions are thought in both algebra and precalculus, analytical geometry are taught in algebra, geometry and precalculus, etc. trigonometry is another one that can be taught alone, or almost any high school math class. I found it somewhat amusing when posters decree that complex number are taught in Algebra 2. What exactly is Algebra 2? Then why are they taught again in precalculus. There’s an apt analogy that math is a spiral, it’s up to the individual how fast one goes around and the breath and depth of the material studied. |
I don't remember if it was algebra 2 or algebra 1, but matrices came up in at least algebra 2, with Cramer's rule and determinants. |
Agreed, precalculus is a collection of topics borrowed from other areas. Still it shows how blurred the demarcation between these classes really is. |
+1. This is a really thoughtful comment. Teaching to the class average is built in, that’s not necessarily wrong because this is the role of the public school, but it should be pretty obvious that the extremes of the mathematical skills distribution won’t benefit much. For a top 1% student you simply need to find alternate forms of instruction , not to prepare for the regular school class, but just to cover what is not taught. But that’s probably true for any subject. |
College algebra has a prerequisites of a high school diploma or GED, and/or remedial Algebra class. A high school diploma has a prequisite of....Algebra 1. GED includes solving quadratics but not imaginary/complex numbers/roots. |
no prep ever if your kid struggles drop down a course
enough with the math arms race. It's pathetic |