O(n^10 + 9n^9 + 20n^8 + 145n^7) == O(n^10)

O(n + 0.001n^3) == O(n^3)

The run time analysis only cares about the largest exponent. If n is going towards infinity the importance of smaller exponents becomes insignificant.

Or to phrase it differently, if we assume the following function:

n + C * n^2

Then for every constant C > 0 that you can imagine I can find an n, so that C * n^2 > n.

No matter how close your C is to 0, the quadratic growth is still superior to the linear growth when looking at large enough n.