1.39. Решите уравнения: a) 4(x + 3) + 5(x - 3) = 105; б) 3(x + 20) - 4x = 10; в) 2x + 3(x + 10) = 380; г) 5(x + 2) - 6(x - 2) = 5.

a) $$4(x + 3) + 5(x - 3) = 105$$ $$4x + 12 + 5x - 15 = 105$$ $$9x - 3 = 105$$ $$9x = 105 + 3$$ $$9x = 108$$ $$x = \frac{108}{9}$$ $$\mathbf{x = 12}$$ б) $$3(x + 20) - 4x = 10$$ $$3x + 60 - 4x = 10$$ $$-x = 10 - 60$$ $$-x = -50$$ $$\mathbf{x = 50}$$ в) $$2x + 3(x + 10) = 380$$ $$2x + 3x + 30 = 380$$ $$5x = 380 - 30$$ $$5x = 350$$ $$x = \frac{350}{5}$$ $$\mathbf{x = 70}$$ г) $$5(x + 2) - 6(x - 2) = 5$$ $$5x + 10 - 6x + 12 = 5$$ $$-x + 22 = 5$$ $$-x = 5 - 22$$ $$-x = -17$$ $$\mathbf{x = 17}$$