For a commutative ring R with identity, dimR shall stand for the Krull dimension of R. It is known that dimR[x]≤2dimR+1. We show that this does not hold for the power series extensions. Using mixed extensions, we construct an example of a finite-dimensional integral domain R such that 2dimR+1<dimR〚x〛<∞. Let D be a finite-dimensional SFT Prüfer domain and D[x1〛⋯[xn〛 be a mixed extension. According to Arnold, dimD[x1,…,xn]=dimD+n and dimD〚x1,…,xn〛=ndimD+1. We generalize Arnold’s result by showing that dimD[x1〛⋯[xn〛=ndimD+1 provided that there is at least one power series extension. In particular, if R=D[x1,⋯,xn−1] and dimD=d>2(n−1)/(n−2), then dimR〚x〛=dn+1>2dimR+1. This is an answer to the question of Coykendall and Gilmer.