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The various other two one-of-a-kind factoring formulas you"ll need to memorize are exceptionally equivalent to one another; they"re the formulas for factoring the sums and the differences of cubes. Here are the two formulas:

You"ll learn in more advanced classes exactly how they came up through these formulas. For currently, simply memorize them.

You are watching: Which expression is a sum of cubes? To aid with the memorization, first notice that the terms in each of the two factorization formulas are exactly the exact same. Then notification that each formula has actually just one "minus" sign. The difference in between the 2 formulas is in the area of that one "minus" sign:

For the difference of cubes, the "minus" sign goes in the direct aspect, ab; for the sum of cubes, the "minus" authorize goes in the quadratic aspect, a2 – ab + b2.

Some civilization use the mnemonic "SOAP" to help store track of the signs; the letters stand also for the direct aspect having the "same" authorize as the authorize in the middle of the original expression, then the quadratic factor beginning with the "opposite" authorize from what remained in the original expression, and also ultimately the second sign inside the quadratic aspect is "always positive".

Whatever before approach finest helps you store these formulas right, usage it, bereason you have to not assume that you"ll be offered these formulas on the test. You must mean to must understand them.

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Note: The quadratic percent of each cube formula does not factor, so don"t waste time attempting to element it. Yes, a2 – 2ab + b2 anda2+ 2ab + b2 variable, however that"s because of the 2"s on their middle terms. These sum- and difference-of-cubes formulas" quadratic terms do not have that "2", and thus cannot factor.

When you"re offered a pair of cubes to aspect, carefully apply the appropriate dominion. By "carefully", I suppose "making use of parentheses to keep track of everything, especially the negative signs". Here are some typical problems:

Factor x3 – 8

This is equivalent to x3 – 23. With the "minus" authorize in the middle, this is a distinction of cubes. To execute the factoring, I"ll be plugging x and 2 right into the difference-of-cubes formula. Doing so, I get: