6) u - v = (f' + f) - (f' - f) = 2f
2f = exp(x) - exp(-x) => (2f)² = exp²(x) - 2exp(x)exp(-x) + exp²(-x)

2f' = exp(x) + exp(-x) => (2f')² = exp²(x) + 2exp(x)exp(-x) + exp²(-x)

f'² - f² = 4exp(x)exp(-x) /4 = exp(x - x) = exp(0) = 1