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Chán ............ !!!

$b) ab(a+b)+bc(b+c)+ca(c+a)+3abc$ $= a^2b + ab^2 + b^2c + bc^2 + a^2c + ac^2 + 3abc$ $= (a^2b + ab^2 + abc) + (b^2c + bc^2 + abc) + (a^2c + ac^2 + abc)$ $= ab(a + b + c) + bc(b + c + a) + ac(a + c + b)$ $= (a + b + c)(ab + bc + ac)$

$1.$ $c) a(b^3−c^3)+b(c^3−a^3)+c(a^3−b^3)$ $= a(b^3 - c^3) - b(a^3 - c^3) + c(a^3 - b^3)$ $= a(b^3 - c^3) - b[(a^3 - b^3) + (b^3 - c^3)] + c(a^3 - b^3)$ $= a(b^3 - c^3) - b(a^3 - b^3) - b(b^3 - c^3) + c(a^3 - b^3)$ $= (b^3 - c^3)(a - b) - (a^3 - b^3)(b - c)$ $= (b - c)(b^2 + bc + c^2)(a - b) -...

It's not worth making him get up early --> There is .....................................