<s>[INST] How to find $\sqrt{1+{4\over x}+{4\over x^2} }$? If   $$abx^2 = (a-b)^2(x+1)$$ then what is $$\sqrt{1+{4\over x}+{4\over x^2} }$$ 

(A) $a+b \over a-b$  (B)$a-b\over a+b$ (C) $a^2+ab$ (d) None

EDIT: What I've done is this:
$$abx^2=(a^2+b^2-2ab)(x+1)$$
=> $$abx^2 - (a^2+b^2+2ab)x - (a-b)^2=0$$
=> $$abx^2 - (a-b)^2x - (a-b)^2=0$$
but this is getting me nowhere.
 [/INST] Ans. is  $\frac{a+b}{a-b}$
As @Tattwamasi Amrutam said find the value of $x$ from
$$abx^2-(a-b)^2x-(a-b)^2=0$$then
$$x=\frac{(a-b)^2 \pm \sqrt{(a-b)^4+4ab(a-b)^2}}{2ab}$$
$$x=\frac{(a-b)^2\pm \sqrt{a^4+b^4-4a^3b-4ab^3+6a^2b^2+4a^3b+4ab^3-8a^2b^2}}{2ab}$$
(I got expansion of $(a-b)^4$ using pascal's triangle)
After the simplification,
$$x=\frac{2a^2-2ab}{2ab}$$
and there is another root that u can find.
For this root only i got
$$\sqrt{1+\frac{4}{x}+\frac{4}{x^2}}=\sqrt{\bigg(1+\frac{2}{x}\bigg)^2}$$ 
$$1+\frac{2}{x}=1+\bigg(\frac{4ab}{2a^2-2ab}\bigg)=\frac{a+b}{a-b}$$
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