Factorise 125 -8a^3 - 27 b^3 - 90 ab
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125 - 8x^3 - 27b^3 - 90ab
= (5 )^3 + (-2x)^3 + (-3b)^3 - 3×5×(-2x)×(-3b)
= (5-2x-3b)( (5)^2 + (-2x)^2 + (-3b)^2 - 5×(-2x) - (-2x)×(-3b) - (-3b)×5)
= (5-2x-3b)(25 + 4x^2 + 9b^2 +10x - 6bx + 15b)
= (5 )^3 + (-2x)^3 + (-3b)^3 - 3×5×(-2x)×(-3b)
= (5-2x-3b)( (5)^2 + (-2x)^2 + (-3b)^2 - 5×(-2x) - (-2x)×(-3b) - (-3b)×5)
= (5-2x-3b)(25 + 4x^2 + 9b^2 +10x - 6bx + 15b)
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