Determine the first two terms, in ascending powers of $u$, of the binomial expansion of

$${\left(1+\frac{3}{4}u\right)}^{-\frac{1}{3}}$$

Hence, in a signal processing application, for small phase offsets $\theta$, show that the windowing function $W(\theta )$ defined by

$$W(\theta )=\cos (5\theta )+{\left(1-\frac{3}{2}{\theta }^2\right)}^{-\frac{1}{3}}$$

can be approximated by $A+B\theta +C{\theta }^2$, where $A$, $B$ and $C$ are constants to be found.