The given sum is what we would get if we were to compute the definite
But we can also compute this directly:
Now by amazing coincidence $9.3$ squared happens to evaluate to
which is good enough to pass for $86.5$. One more lucky coincidence: $9.3$ is easily multiplied by $\frac 23$. So we need to do $6.2$ times $86.5$ which yields
The $.3$ cancels ...