Factorise :
a2 - 23a + 42
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= a2 - (21 + 2)a + 42
= a2 - 21a - 2a + 42
= (a2 - 21a) - (2a - 42)
= a(a - 21) - 2(a - 21)
= (a - 21)(a - 2)
Hence,a2 - 23a + 42 = (a - 21)(a - 2)
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