Answer:
Hello,
Step-by-step explanation:
[tex]u_2=u_1*r\\u_3=u_1*r^2\\u_4=u_1*r^3\\\\\dfrac{u_4}{u_3} =r=\dfrac{-56}{28} =\boxed{-2}\\\\u_1=\dfrac{u_3}{r^2} =\dfrac{28}{(-2)^2} =\dfrac{28}{4}=\boxed {7}\\\\\\\displaystyle\ s=\sum_{i=1}^6\ u_i\\\\=u_1*\sum_{i=1}^6\ r^{i-1}\\\\=7*\dfrac{(-2)^6-1}{-2-1} \\\\=-7*\dfrac{65}{3} \\\\\\\boxed{s=-\dfrac{455}{3} }\\[/tex]
