Taking Every Other Term

$\text {Solution:}$

I too did it the same way as Muhammad Maulana but for the sake of variety here is a small trick.

Note that two options repeats twice so both of them are not the solutions since only one option is correct.

$a \approx 1.6$ $(\frac{\pi^2}{6})$ .

$\frac{a}{3}-1$ is negative $(\approx -0.46)$ but the required expression is a sum of positive terms leaving us with the only option $\boxed{\dfrac{3a}{4}-1}$