List of variables as function arguments

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I would like to define a function that takes a long list of variables as input and some complicated expression of my variables as my output, but I just don't seem to be able to make it work.

Something along the line of:

var('x_1,x_2,x_3')
long_list_of_variables = [x_1, x_2,x_3]
complicated_expression=(x_1^2+x_2^2-x_3)^2
f(tuple(long_list_of_variables))=complicated_expression

But with much more variables.

I've tried different variations of this but I always seem to end up with the 'can't assign to function call'-error. Are there any way to achieve what I'm trying to do?