In the book
Introduction to Algorithms (3th edition) chapter 7
the recurrence of the running time of quicksorts partitioning is given by $$T(n) = T(n-1) + \Theta(n)$$ as the worst-case happens when the partitioning has one subproblem with $n-1$ elements and one with $0$ elements.
Now, they they guess $T(n)=\Theta(n^2)$ for the substitution method. I don't understand this, why choose $\Theta(n^2)$? I personally would have chosen $\Theta(n)$, which is wrong.