А) 6х(х+1)-5х(2-х) = 6x^{2} + 6x - 10x + 5x^{2} = 11x^{2} - 4x
б) (а-4)(а-3)-(а+4²) = (а-4)(а-3)-(а+16) = a^{2} - 3a - 4a + 12 - a - 16 = a^{2} - 8a - 4
в) 20n+5(2-n)² = 20n+5 (2-n) (2-n) = 20n + 5 (4-2n-2n+n^{2}) = 20n + 5(4-4n+n^{2}) = 20n + 20 - 20n + 5n^{2} = 20 + 5n^{2}