x² = x + 1
x^5 = x·x²·x² = x(x+1)² = x(x² + 2x + 1) = x(x + 1 + 2x + 1)
= 3x² + 2x = 3(x + 1) + 2x = 5x + 3
所以
x^5 < 5x + 4