Showing that a function is continuous everywhere.

$$f(x)=\lvert x^2 + 2x - 3\rvert$$

In the above function we can show continuity at a point by finding the left hand and right hand limits at that point! But how do we show that, this function is continuous at everywhere?

• Polynomials are always continuous everywhere. A composition of two continuous functions leads to a continuous function. – Andrew Li Apr 1 '18 at 0:45

The composition of two functions that are continuous everywhere is continuous everywhere. Since $|\cdot|$ and $x^2+2x−3$ are continuous everywhere, and your $f(x)$ is the composition of $|\cdot|$ and $x^2+2x−3$, you can conclude.
• @TobyMak - It is a continuous function for any $x \in \mathbb{R}$. – Taroccoesbrocco Apr 1 '18 at 0:27
Note that $$f(x)=\lvert x^2 + 2x - 3\rvert$$
is the composite function of the absolute value function and the polynomial function $$x^2 + 2x - 3$$ which are continuous everywhere.
So, as you can show continuity of $f(x)=\lvert x^2 + 2x - 3\rvert$ at each point of its domain (using limits for example), you can conclude that your function is continuous "everywhere".