# summation – Trigonometric Series Function

I came across a function where we have to find n where they give us two separate values a and b.

The function is:

f (n) = 2 x $$7^ {n/2}$$ x cos(nθ) + n x $$4^n$$ , where θ = $$tan^{-1}$$ ($$frac32$$)

=> the 3 is square rooted in the theta function, I couldn’t figure out how to do that with Math Jax.

The summation is:

$$sum_{n=a}^{n=b} f(n)$$

With the values of a and b, I need to figure out how to get the value of n given by the function and series.