if P + Q + R = 21 and PQ + QR + PR = 21 and pqr = 2 - 1 find the value of P Cube + Q Cube + R cube

if P + Q + R = 21 and PQ + QR + PR = 21 and pqr = 2 - 1 find the value

1.	if P + Q + R = 21 and PQ + QR + PR = 21 and pqr = 2 - 1 find the value of P Cube + Q Cube + R cube
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