А^3 + b^3 = ( a + b )( a^2 - ab + b^2 )
a^3 + b^3 = 72
a + b = 6
72 = 6( a^2 - ab + b^2 )
a^2 - ab + b^2 = 72 : 6
a^2 - ab + b^2 = 12
a^2 + b^2 = ( a + b )^2 - 2ab
a + b = 6
( a + b )^2 = 36
a^2 - ab + b^2 = 36 - 2ab - ab = 36 - 3ab
36 - 3ab = 12
3ab = 36 - 12
3ab = 24
ab = 8
Ответ 8