Find the Value

\[ a-b=4 \]

\[ ab=21 \]

Find:

\[ a^3-b^3 \]

Solution:

Using identity:

\[ a^3-b^3=(a-b)^3+3ab(a-b) \]

\[ a^3-b^3 = (4)^3+3(21)(4) \]

\[ = 64+252 \]

\[ =316 \]

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