While assuming the terms we should keep one thing in our mind that assumption of the terms should be such which makes the calculations easy.

Three terms \(\frac{a}{r},a,ar\,\)

Four terms \(\frac{a}{{{r^3}}},\frac{a}{r},ar,a{r^3}\)

Five terms \(\frac{a}{{{r^2}}},\frac{a}{r},a,ar,a{r^3}\)