Match each polynomial to the term that does not change the polynomial's classification when added to it.

1) If we add - x to x² + 3 x we will get: x² + 3 x - x = x² + 2 x, with same number of terms and same degree (2),

2) xy - 1 + xy = 2 xy - 1

3) x + y - 3 y = x - 2 y

4) x³ - 5 x² + 1 - 2 x³ = - x³ - 5 x² + 1

Pairs:

(x² + 3 x) ↔ (-x)

(xy - 1) ↔ (xy)

(x + y) ↔ ( - 3 y)

(x³ - 5 x + 1) ↔ ( - 2 x³)