<s>[INST] Small angle approximation on cosine The problem is

Using the small angle approximation of cosine, show that $3-2\cos(x)+4\cos^2(x)\approx 5-kx^2$ where k is a positive constant

I did solve it by using $\cos^2(x)=1-\sin^2(x)$ on the $\cos^2(x)$, by plugging $\sin^2(x)\overset{x\to 0}{\approx}x^2$ and $\cos(x)\overset{x\to 0}{\approx}1-\frac{x^2}{2}$ to get $$3-2(1-\frac{x^2}{2})+4(1-x^2)=5-3x^2$$ hence $k=3$. But why does using $\cos^2(x)\overset{x\to 0}{\approx}(1-\frac{x^2}{2})^2$ doesn't work out? I originally tried plugging that into the $\cos^2(x)$ but got another complete thing. why?
 [/INST] Directly subbing $\cos^2x=(1-x^2/2+\cdots)^2$ should work out, provided you expand properly:
$$\cos^2x=1-2(x^2/2)+\dots=1-x^2+\cdots$$
$$1-\sin^2x=1-(x-\cdots)^2=1-x^2+\cdots$$
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