[tex]b_5=b_1*q^4=5*(-2)^4=80[/tex]

[tex]\displaystyle\bf\\b_{1} =5\\\\q=-2\\\\\\b_{5} =b_{1} \cdot q^{4} =5\cdot (-2)^{4} =5\cdot 16=80\\\\Otvet: \ b_{5} =80[/tex]