Szczegółowe wyjaśnienie:
[tex]19. \\ (2a + 6b) + (6a - 2b) = 8a + 4b \\ (10m + 15n) - (6m - 6n) = 4m + 21n \\ \\ 20. \\ 1.(x + 3)(x - 5) = x {}^{2} - 5x + 3x - 15 = x {}^{2} - 2x - 15 \\ 2.(5 - x)(2x - 3) = 10x - 15 - 2x {}^{2} + 3x = - 2x {}^{2} + 13x - 15 \\ 3.(x + 3)(x + 5) = x {}^{2} + 5x + 3x + 15 = x {}^{2} + 8x + 15 \\ 1.b \\ 2.c \\ 3.a[/tex]