4) a) $$-13 \cdot 26^{-1}$$; б) $$-6 \cdot 3^{-3}$$; 5) a) $$81 \cdot 3^{-5}$$; б) $$16 \cdot (2^{-3})^2$$

4.

1. a) \( -13 \cdot 26^{-1} = -13 \cdot \frac{1}{26} = -\frac{13}{26} = -\frac{1}{2} \)
2. б) \( -6 \cdot 3^{-3} = -6 \cdot \frac{1}{3^3} = -6 \cdot \frac{1}{27} = -\frac{6}{27} = -\frac{2}{9} \)

5.

1. a) \( 81 \cdot 3^{-5} = 3^4 \cdot 3^{-5} = 3^{4+(-5)} = 3^{-1} = \frac{1}{3} \)
2. б) \( 16 \cdot (2^{-3})^2 = 2^4 \cdot 2^{-3 \cdot 2} = 2^4 \cdot 2^{-6} = 2^{4+(-6)} = 2^{-2} = \frac{1}{2^2} = \frac{1}{4} \)