In order to get the common denominator when adding fractions or subtracting fractions, you have to make sure that the denominator of the two items are divisible by each other. In this case, 9 is not divisible by any of the given denominators which is 4 and 5. So in this case, the common denominator will have to be 20. 5 x 4 will equal to 20 and you will also multiply 3 x 5 to get the new numerator. After that, you will multiply 2 x 4 to get the new numerator of the other fraction. The new fractions will be 15/20 and 8/20. From there, you can add or subtract
The sum must be larger (not equal to) the longest side. If it's exactly the same, there's no room to for the shorter two sides to angle up to form a triangle.
