Hi,
Oh, yes you are right 1-(.95^8)
Because you want at least one test positive ,it means 1 positive or 2 positive or 3 positive,... .
(.95^8) means all cases are negative and 1-(.95^8) means you calculate all situations except all negative.
You can't use 0.5^8 because this means all tests positive not at least one positive.