Factorise :
1 - 18x - 63x2
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= 1 - (21 - 3)x - 63x2
= 1 - 21x + 3x - 63x2
= (1 - 21x) + (3x - 63x2)
= 1(1 - 21x) + 3x(1 - 9x)
= (1 - 21x)(1 + 3x)
Hence,1 - 18x - 63x2 = (1 - 21x)(1 + 3x)
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