The radius of ${\mathrm{La}}^{3+}$ (at. no. 57) is $1.06{\mathrm{A}}^{\mathrm{o}}$ What may be the radius of ${\mathrm{Lu}}^{3+}\left(\mathrm{at}.\mathrm{no}.71\right)?$

Due to lanthanoid contraction the decrease in the ionic radii among the lanthanides  is very small.