<s>[INST] Assuming x is small, expand $\frac{\sqrt{1-x}}{\sqrt{1+2x}}$ up to and including the term in $x^{2}$ I have tried this many times but can't quite land on the correct answer.
The correct answer:
$1-\frac{3x}{2}+\frac{15x^{2}}{8}$
These are the steps I took:

*

*Re wrote it as: $\left ( 1-x \right )^{\frac{1}{2}}\left ( \left ( 1+2x \right )^{\frac{1}{2}} \right )^{-1}$


*Using bionmal expansion I expanded $\left ( 1-x \right )^{\frac{1}{2}}$ up to $x^{2}$



*

*For this I get : $\left (1-\frac{x}{2}-\frac{x^{2}}{8}  \right)$


*Then to solve the second bracket written in step 1 I did $\left (1+2x  \right )^{\frac{1}{2}}$


*

*For this I got $\left (1-x+\frac{x^{2}}{4}  \right)$


*Then finally I raised this to the power one so $\left (1-x+\frac{x^{2}}{4}  \right)^{-1}$


*For this I got $\left (1+x+\frac{3x^{2}}{4}  \right)$


*Then I just expaned those two brakcets so: $\left (1-\frac{x}{2}-\frac{x^{2}}{8}  \right)$$\left (1+x+\frac{3x^{2}}{4}  \right)$
Once expanded, I then neglected powers bigger than $x^{2}$ (mentioned in question). Then collected like terms. However either my method or the algebra is going wrong, and I just need some help with this.
 [/INST] $$\frac{\sqrt{1-x}}{\sqrt{1+2x}}=(1-x)^{1/2}(1+2x)^{-1/2}$$
Consider that $(1+t)^{\alpha}\sim 1+\alpha t+\frac{1}{2}(\alpha -1)\alpha t^2$, hence you obtain:
$$ (1-x)^{1/2}\sim 1-\frac{x}{2}-\frac{x^2}{8} \quad \text{and}\quad (1+2x)^{-1/2}\sim 1-x+\frac{3x^2}{2}$$
It follows that:
$$\left(1-\frac{x}{2}-\frac{x^2}{8}\right)\left(1-x+\frac{3x^2}{2}\right)\sim 1-x+\frac{3x^2}{2}-\frac{x}{2}+\frac{x^2}{2}-\frac{x^2}{8}=1-\frac{3x}{2}+\frac{15x^2}{8}$$
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