a) $\sqrt[]{13^{2}-12^{2}}$
= $\sqrt[]{(13-12)(13+12)}$
= $\sqrt[]{1.25}$
= $\sqrt[]{25}$
= 5
b) $\sqrt[]{17^{2}-8^{2}}$
= $\sqrt[]{(17-8)(17+8)}$
= $\sqrt[]{9.25}$
= $\sqrt[]{9}$ .$\sqrt[]{25}$
= 3.5
= 15
c) $\sqrt[]{117^{2}-108^{2}}$
= $\sqrt[]{(117-108)(117+108)}$
= $\sqrt[]{9.225}$
= $\sqrt[]{9}$ . $\sqrt[]{225}$
= 3.15
= 45
