Because when your solving a quadratic equation your looking for x-intercepts which is where why equals 0 and x equals what ever the answer is.
r=0 is the solution...
y = 0" the answer is YES.
A linear equation.
Yes, the equation has a slope of 0.
The equation 0 equals 0 is an identity and contributes absolutely nothing to the part of the graph that you should shade or not. The tautological statement can be ignored.
It's an equation in 'N' ... about the simplest one you could write.The solution of that equation is: N=0 .
0 = 0 is an identity and not an equation. Equations have solutions, identities do not.