Rectangles and Poles: What's the Difference?

Algebra Level pending

There exists a set of points such that their rectangular coordinates ( x , y ) (x, y) and their polar coordinates ( r , θ ) (r, \theta) are the same. This set of points lies on a function that can be written as y = a x 2 + b x + c y=ax^{2}+bx+c . What is a + b + c a+b+c ?

e e 0 0 1 1 π \pi

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1 solution

Ved Pradhan
Jun 24, 2020

If we call the rectangular coordinates ( x , y ) (x, y) and the polar coordinates ( r , θ ) (r, \theta) , we get that x = r x=r and y = θ y=\theta . Then, let’s use some rectangular polar conversion formulas.

x = r cos ( θ ) x=r\cos(\theta) x = x cos ( θ ) x=x\cos(\theta) cos ( θ ) = 1 \cos(\theta)=1 θ = 2 π × n \theta=2\pi \times n

Now, remember that an equation in the form θ = c \theta=c is a line that goes through the origin, and an equation in the form y = c y=c is a line that has a slope of zero. Because both of these equations need to result in the same line in this case, the only line that works is y = θ = 0 y=\theta=0 , where the constant c = 0 c=0 .

Wait, but what’s that you say? I made a mistake? Where? Aha! You have good eyes. In our work above, we divided by the variable x x . But what if x = 0 x=0 ? Well, then, let’s use our other conversion formula.

y = r sin ( θ ) y=r\sin(\theta) y = x sin ( θ ) y=x\sin(\theta) y = 0 × sin ( θ ) y=0 \times \sin(\theta) y = 0 y=0

Again, we get to the same result, so we don’t have to change anything. We know that the equation of the line is y = 0 y=0 , which can be written as y = 0 x 2 + 0 x + 0 y=0x^{2}+0x+0 . Thus, a = b = c = 0 a=b=c=0 , and therefore, a + b + c = 0 a+b+c=\boxed{0} .

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