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Need help on this algegra question:?
4a^2-21-5a. Need to factor. Please show me as I cant remember how.
1 Answer
- 10 years agoFavorite Answer
4a^2 - 21 - 5a = 4a^2 - 5a - 21 = (a ....)* (4a .....) ----->because it cannot factor as (2a...) (2a....), as this would have given an even number of "middle term"..
Finally, trial and error will give the correct factoring as = (a -3)*(4a +7).... Check that this will give 4a^2 when foiled, as well as -21 as the constant (last) term... However, MOST IMPORTANTLY, this will give +7a - 12a... or, -5a as the middle term.
So:
4a^2 - 21 - 5a = (a -3)*(4a +7)
Thanks, hope that helps.
