Is $\delta \_{mn}=1$ when $m\neq n$, and $\delta \_{mm}=0$?

I am not very good at Math. So would you give me the answer and explanation please?
['https://math.stackexchange.com/questions/846695', 'https://math.stackexchange.com', 'https://math.stackexchange.com/users/135962/']
You got it. [For example, this.](http://www.wolframalpha.com/input/?i=sin%20300) What is your worry?
$$\cos^2\theta + \sin^2\theta = 1^2$$
$$r^2(\cos^2\theta + \sin^2\theta) = r^2(1^2)$$
$$(r\cos\theta)^2 + (r\sin\theta)^2 = r^2$$

$$x^2+y^2 = r^2$$

Draw a circle of radius 2. For what values of $\theta$ would the y-coordinate of the circle $x^2+y^2=2^2$ intersect the line $y = -\sqrt{3}$?

![enter image description here](https://i.stack.imgur.com/j0ivq.png)

Hint #2

What does $x$ equal? What is the distance between $(-x, -\sqrt{3})$ and $(x,-\sqrt{3})$?