21. (a + b)^3 -a-b (class 9)
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- (a + b)³ - a - b
We know that (a + b)³ = a³ + b³ + 3a²b + 3ab² :
=> (a³ + b³ + 3a²b + 3ab²) - a - b
=> a³ + b³ + 3a²b + 3ab² - a - b
=> a³ + b³ + 3ab(a + b) - (a + b)
Hence, answer is a³ + b³ + 3ab(a + b) - (a + b).
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