When you add three of something to four of something, you get seven of something. It doesn't matter what the somethings are. Think of ninths as somethings. If you add three ninths and four ninths, you have to wind up with seven ninths. You can't wind up with seven eighteenths, because you weren't adding eighteenths to begin with.
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Yes you have to add with the same denominator. when ever you do fractions they have to have the same denominator no matter what. So thats a yes
if it has a denominator
no, to add and subtract like and unlike fractions the denominator has to be the same,
numerators you add, denominators you leave it the same
You can add or subtract fractions only if they are "like" fractions, that is, only if they have the same denominator - unless you know your fractions really well.