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Question

Without actually calculating cubes find the following (0.2)³-(0.3)³+(0.1)³

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Solution

using identity:
x3 + y3 + z3 - 3xyz = (x + y + z)(x2 + y2 + z2 – xy – yz – zx)

if x + y + z = 0,

then, x3 + y3 + z3 - 3xyz = 0

or x3 + y3 + z3 = 3xyz

(0.2)3 - (0.3)3 + (0.1)3 can be written as
(0.2)3+ (-0.3)3 + (0.1)3 //since (-x)3 = -x3 //

here, (0.2) + (-0.3) + (0.1) = 0.3 - 0.3 = 0

so, (0.2)3 - (0.3)3 + (0.1)3 =

3*(0.2)(-0.3)(0.1) = - 0.018

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