f(x) = cos(x + φ)
f'(x) = -sin(x + φ)
f(x) + f'(x) = cos(x + φ) - sin(x + φ)
= √2[(√/2)cos(x + φ) - (√/2)sin(x + φ)]
= √2[sin(π/4)cos(x + φ) - cos(π/4)sin(x + φ)]
= √2sin(π/4 - x - φ)
= -√2sin(x + φ - π/4)
f(x) + f'(x)是奇函数, 则φ - π/4 = kπ, k为整数
φ∈(0,π)内的解为π/4 (k = 0)
f(x) = ax³ + lnx定义域: x > 0
f'(x) = 3ax² + 1/x = 0
x = -1/³√(3a) > 0
a < 0