Mathematics - Sequence & Series Question with Solution | TestHub

Write $a,b,c,d=a,ar,a{r}^2,a{r}^3$. The product is $a^4{r}^2{\left(1+r^2+r^4\right)}^2$, which is ${\left[a^{2}r\left(1+r^2+r^4\right)\right]}^2=(ab+bc+cd{)}^2$.