Translation:Studies have been using different methods.
I do not understand when to translate with "have used" or "have been using". It appears that sometimes duolingo accepts both forms, but sometimes it does not
DL accepts both because, in reality, Portuguese doesn't have the form of present perfect that we use in English, so it's sometimes translated as progressive and other times as past or simple present.
This is as complete as you are going to find online:
The standard translation is:
Have used = usou (yes, the simple Portuguese past is the best tense for English present perfect)
Have been using = tem usado (that is surely the first idea that comes to a Brazilian's mind)
Those rules are only changed by special sentences containing certain adverbs that twist time understanding. Perhaps some special verbs too.
Like some English verbs that cannot be used with a continuous form, I think. Then you would use present perfect in English.