<s>[INST] Show that $\log(1+y) \approx y- \frac {y^2}2 + \cdots$ without Taylor Series 
For small $y$, prove that $\log(1+y)\approx y -\frac {y^2}2 + \cdots $

I have no idea to solve it.
 [/INST] One approach directly based on a definition of $\log x$ is as follows. We use the following definition of $\log x$ $$\log x = \lim_{n \to \infty}n(x^{1/n} - 1)$$ Replacing $x$ by $(1 + x)$ and for $|x| < 1$ using binomial theorem we get
\begin{align}
\log (1 + x) &= \lim_{n \to \infty}n((1 + x)^{1/n} - 1)\notag\\
&= \lim_{n \to \infty}n\left(\frac{x}{1!}\frac{1}{n} - \frac{x^{2}}{2!}\frac{1}{n}\left(1 - \frac{1}{n}\right) + \frac{x^{3}}{3!}\frac{1}{n}\left(1 - \frac{1}{n}\right)\left(2 - \frac{1}{n}\right) + \cdots\right) \notag\\
&= \lim_{n \to \infty}\left(x - \frac{x^{2}}{2!}\left(1 - \frac{1}{n}\right) + \frac{x^{3}}{3!}\left(1 - \frac{1}{n}\right)\left(2 - \frac{1}{n}\right) + \cdots\right)\notag\\
&= x - \frac{x^{2}}{2} + \frac{x^{3}}{3} - \cdots\notag
\end{align}
The last step where I took limit of an infinite series term by term requires some justification which might not be suitable at your stage of learning (as guessed from the content of question and comments to it). Moreover since you need only an approximation it is better to just take the terms till $x^{2}$ in the binomial expansion and then take limit.
Another approach is available here.
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