$I f p = 15, q = - 8, r = - 7, p^{3} + q^{3} + r^{3} =$ ___ .

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Solution

$(p + q + r) (p^{2} + q^{2} + r^{2} - p q - q r - r p) = p^{3} + q^{3} + r^{3} - 3 p q r . H e r e, p + q + r = 0. S o, p^{3} + q^{3} + r^{3} = 3 p q r$ .

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