How to treat small number within square root guys.I am reading a math book. It has a equation shown as follows,
$\sqrt{(1+\Delta^2)}$
And then,since $\Delta$ is very small, it can be written as,
$\sqrt{(1+\Delta^2)} = (1+\frac12\Delta^2)$
What is the theory behind this equation?

Square both sides to get $1+\Delta^2\approx 1+\Delta^2+\frac{1}{4}\Delta^4$
which is a close approximation because when $\Delta$ is very small, $\frac{1}{4}\Delta^4$ is close to $0$.
