How to implement generalization of a formula and call values from dictionaries in Python

Question

I have the following formula:

y = a1cos(θ1) +a2cos(θ1+θ2) +...+ancos(θ1+...+θn)

The goal is to implement it in Python so it works with any number of a-value/theta angles combination.

So far I have written code which stores in two separate dictionaries:

1. A combination of a-value number and a value
2. A combination of a-value number and an angle value

This is implemented via two for loops where i is the number of values the user wants the formula to work with. Here is the code:

a_values_list = {}
a_number = int(input("\nHow many values of a would you like to enter?"))


for i in range(0, a_number):
    key = input("Input a-value number")
    value = input("Input corresponding value of a")
    a_values_list[key] = value


angles_list = {}

for i in range(0,a_number):
    key = input('Input a-value number')
    value = input("\n Enter a corresponding angle displacement between 0 and pi")
    angles_list[key] = value
 

I need help with the following:

1. Figuring out how to implement the formula based on the number of a-values/angle combinations stored in the dictionaries
2. Getting the angle values from the dictionaries and adding them within the cos statements

Thank you for your time