a+b= 4
a в кубе + b в кубе = 40
ab=?
a^3+b^3=(a+b)(a^2-ab+b^2)=4(a^2-ab+b^2)=40
(a^2-ab+b^2)=10
(a^2+b^2)=(a+b)^2-2ab
(a+b)^2-2ab-ab=10
16-3ab=10
ab=2
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a^3+b^3=(a+b)(a^2-ab+b^2)=4(a^2-ab+b^2)=40
(a^2-ab+b^2)=10
(a^2+b^2)=(a+b)^2-2ab
(a+b)^2-2ab-ab=10
16-3ab=10
ab=2