I have equation with default $4$ terms but number of terms are related to input
for example :
Say the equation is $X - x^2 + x^3 - x^4$
If my input was $1$ then the equation equals $X$
If it was $2$ equation equals $X - x^2$ .
If it was $3$ then equation will equal $X- x^2 +x^3$ and so on.
I thought of making equations that have same result (say $1$) for specific numbers like (say $3$, $4$) and results zero for $2$ so I can put this equation on the second term and so on with other terms but how to make such an equation?
Is there a mathematical way to do that
Thanks in advance
You can define $f(n)=X-\sum_{i=2}^n (-1)^{i-1}x^i$ Having the upper limit of the sum depend on $n$ can make the number of terms depend on $n$.