Let the number be xy
x - y = 9
Product of x and y = x(x - 8)
P =x2 - 8
$\frac{dp}{dx}=2x - 8$
For p max
$\frac{dp}{dx}=0$
i.e. 2x - 8 = 0
x = 4
y = -4
raphael answered the question on January 16, 2017 at 12:23
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