@DanielFischer Could I ask you something?
We have the following pseudocodes:

 quicksort(A,p,r)
     if p<r then
        q<-partition(A,p,r)
        quicksort(A,p,q-1)
        quicksort(A,q+1,r)



   partition(A,p,r){
      x<-A[r]
      i<-p-1
      for j<-p to r-1
           if A[j]<=x then
              i<-i+1
              swap(A[i],A[j])
      swap(A[i+1],A[r])
      return i+1

I want to show that if A contains distinct elements and is sorted in decreasing  order, then the execution time of quicksort is $\Theta(n^2)$.