Factorise the following:
20x2 - 45y2
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20x2 - 45y2 = 5(4x2 - 9y2) = 5[(2x)2 - (3y)2].
Using identity,
a2 - b2 = (a + b)(a - b).
5[(2x)2 - (3y)2] = 5(2x + 3y)(2x - 3y).
Hence, 20x2 - 45y2 = 5(2x + 3y)(2x - 3y).
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