What is the correct first step when completing the square for the expression $x^2 + 5y^2 - 10x + 20y - 1$?

Group the terms by variable: $(x^2 - 10x) + (5y^2 + 20y) - 1$.

Factor out 5 from the entire expression.

Add and subtract $(10/2)^2$ to the x-terms.

Treat x as a constant and work only with the y-terms.

Quadratic Optimization Practice — Grade 4 amc_math