Approximation for -1 exponent expression

PhyPsy
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How do you come up with this approximation?

[1+H(t-t_0)- \frac{1}{2}qH^2(t- t_0)^2]^{-1}\approx1+ H(t_0-t)+ \frac{1}{2}qH^2(t-t_0)^2+ H^2(t-t_0)^2

Is there a rule that leads to this approximation?
 
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Use the Binomial theorem.

1/(1 + x) = 1 - x + x^2 ...
 

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