a) $\left(-5\right)\cdot \left(-5\right)\cdot \left(-5\right)\cdot \left(-5\right)\cdot \left(-5\right)={\left(-5\right)}^5<0$

b) $4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4=4^{10}>0$ $4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4\cdot 4=4^{10}>0$

c) $\frac{4}{5}\cdot \frac{4}{5}\cdot \frac{4}{5}\cdot \frac{4}{5}\cdot \frac{4}{5}\cdot \frac{4}{5}\cdot \frac{4}{5}={\left(\frac{4}{5}\right)}^7>0$

d) $\left(-1\right)\cdot \left(-1\right)\cdot \left(-1\right)\cdot \left(-1\right)={\left(-1\right)}^4>0$

e) $2,8\cdot 2,8\cdot 2,8\cdot 2,8={2,8}^4>0$

f) $\left(-1\frac{1}{2}\right)\cdot \left(-1\frac{1}{2}\right)\cdot \left(-1\frac{1}{2}\right)={\left(-1\frac{1}{2}\right)}^3<0$