Use the identity $(x + a) (x + b) = x^{2} + (a + b) x + a b$ to find the following products:
$(4 x + 5) (4 x - 1)$ .

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Solution

$(4 x + 5) (4 x - 1) = (4 x)^{2} + (5 - 1) \times 4 x + 5 \times (- 1)$ (1 mark)
$[U s i n g i d e n t i t y (x + a) (x + b) = x^{2} + (a + b) x + a b]$
$= 16 x^{2} + (5 - 1) \times 4 x - 5$ (0.5 mark)
$= 16 x^{2} + 4 \times 4 x - 5$
$= 16 x^{2} + 16 x - 5$ (0.5 mark)

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