Question 19

If $$x + y = 3$$, then what is the value of $$x^3 + y^3 + 9xy$$?

Solution

x+y=3
Cubing on both sides
$$x^{3}+3xy(x+y)+y^{3}$$=27
$$x^{3}+3xy(3)+y^{3}$$=27
$$x^{3}+9xy+y^{3}$$=27

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