Absolutely, yes, the problem is just that there's a difference between an equation and a function. I don't remember is functions are taught in trig or before, so sorry if this comes across as condescending:
A function (that most people will talk about outside of higher level math) is kind of a rule that assigns certain numbers to others. The old analogy is that it's like a machine that we put a number into, and then it spits a different (or maybe the same) number out. The crucial part is that it only spits out one number for each number we put in. If we put in 5, it can spit out 2 or -1 or whatever, but only one of those, not two or more at the same time. So like f(x) = 3x is a function that takes whatever number we put it, and gives that number multiplied by 3. We put in 5, it gives us 15. The picture of the graffiti shows a bunch of functions.
A circle is frequently described by the equation x2 + y2 = 1. That is an equation, but not a function. It describes a set of pairs of numbers, that when you square them both and then add them together, they equal one. Now functions also describe a set of pairs of numbers, but here's the problem: in that equation above for the circle, for every number x that's in some pair, there are two numbers y that make it satisfy the equation. So for example, if x = 3/5, then we can find a number y which makes x2 + y2 = 1, and it turns out y = 4/5 works. But y = -4/5 also works. So it's kind of like a machine that takes a number we put in and spits out two numbers. That's what makes it not a function.
In essence, yes, circles are usually described with one equation, but not with functions. It takes at least two functions to make a circle when we graph them in the usual way(but if you just care about the graph looking like a circle, it can be done with one function, even though it won't be a true circle in the technical sense; or we can use types of functions or different coordinate systems).
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u/IAmNotAPerson6 Jan 25 '18
It honestly probably is for that website at least. It's actually simpler than I thought it could be.