shouldn't this be translated as passive form? "This formula is easy to be used?"

"this formula is easily used"

i'm confused as to why this is incorrect?

'Ee' sound at the end of 'dipakai' should sound more shorter than this.

It may not be a strict translation of the word "dipakai"; but in this context (i.e. science), "to apply" should be aceptable, hence "This formula is easy to apply."