\(3^{1993}=\left(3^{498}\right)^4.3=\left(.....1\right).3=\left(.....3\right)\)
\(17^{1000}=\left(17^{250}\right)^4.17=\left(.....1\right).17=\left(.....17\right)\)
\(39^{7521}=\left(39^{7520}\right)^4.39=\left(.....1\right).39=\left(.....39\right)\)