human
help me solve
f[i = kf[i-1 + ka,
where '1' can be replaced by variable
gpt
I'm not sure if I am able to interpret the formatting of the given equation correctly. Also, you should mention for which variable you want to solve. 

Assuming you meant
f(i) = k f(i-1) + k a
then this is a recurrence relation.
It is usually used in tandem with a starting condition for, e.g, f(0).
For example, if f(0) = a, then the next three results are:
f(1) = 2 k a
f(2) = 2 k^2 a + k a
f(3) = 2 k^3 a + k^2 a + k a

If the decrement 1 is changed to another value, other starting conditions might be required, e.g. for f(1).

Have I interpreted your question correctly?
human
No, I’m looking for a similar type of function where the decrement value could be replaced with another variable
gpt
I apologize for misunderstanding your question. 
Let me explain how the function f(i) = k f(i-1) + k a can be written as follows:
f(i) = k (f(i-1) + a) = k(k (f(i-2)+ a ) + a) = k (k (k (f(i-3)+ a ) + a) + a) …
Therefore, we have: f(i,t)= (k ^ t) f(i-t) + a k + a k^2 + … + a k^t
The second term of this function is a geometric series minus its first term, where a is the first term and k is the common ratio.
For a geometric series, the sum is: S(n)=b (k ^ n -1) / (k-1), where b is the first term, k is the common ratio, and n is the number of terms.
So the second term of the above function can be written as: a k + a k^2 + … + a k^t = S(t+1) - a=(a (k ^ (t+1) -1) / (k-1))-a
Hence, the final result is: f(i,t) =(k ^ t) f(i-t) + (a (k ^ (t+1) -1) / (k-1))-a.
We can verify this by plugging in t = 1, and we get: f(i,1) = k f(i-1) + a k
This matches the original function.
So the final answer is: f(i,t) =(k ^ t) f(i-t) + (a (k ^ (t+1) -1) / (k-1))-a.