There's a mnemonic for keeping track of the signs. The factoring formulas are:
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You've noticed that the terms inside the formula are always the same; it's only the signs that differ. Use "SOAP" to keep track of the signs:
Take the sign of the sum or difference of cubes that you're needing to factor. Then look at the two parentheses that you need to fill in:
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The letters stand for "the Same sign as what you're factoring", "the Opposite sign from what you're factoring", and "Always use a Plus sign here".