Factorise :
x2 + 5xy + 4y2
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= x2 + (4 + 1)xy + 4y2
= x2 + 4xy + 1xy + 4y2
= (x2 + 4xy) + (1xy + 4y2)
= x(x + 4y) + y(x + 4y)
= (x + 4y)(x + y)
Hence,x2 + 5xy + 4y2 = (x + 4y)(x + y)
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