Question 72

The coefficient of x in $$(x - 3y)^{3}$$ is :

Solution

$$(x - 3y)^{3} = x^3 - (3y)^3 - 3x.3y(x - 3y)$$

($$(a - b)^3 = a^3 - b^3 - 3ab(a - b)$$)

= $$x^3 - 27y^3 - 9xy(x - 3y)$$

= $$x^3 - 27y^3 - 9x^2y - 27xy^2$$

The coefficient of x = 27$$y^2$$

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