Find the first term of a GP if the third term i... - SS2 Mathematics Sequence and Series Question

Assume the 3rd term is the geometric mean of the 1st and 6th terms

$$Geometricmean,GM=\sqrt{al}$$

$$72=\sqrt{a\times 243}$$

$${72}^2=243a$$

$a=\frac{{72}^2}{243}=21.3$