Hello, Fnus!

We are told: .

Let be a point between the two sources.

. . Let = distance from "40".

. . Then = distance from "5".

The total intensity is: .

Then we have: .

Multiply by

This simplifies to: .

. . which factors: .

. . and has the real root: .

Since is always positive,

. . the function is alwaysconcave up.

Therefore, provides minimum