轻松掌握cosx的二倍角公式，让你的数学学习更上一层楼！

cosx的二倍角公式是三角函数中的一个重要公式，它可以帮助快速计算与cos(2x)相关的表达式。这个公式不仅在数学学习中非常有用，而且在物理、工程和其他科学领域中也非常实用。

二倍角公式的定义：

cos(2x) = 1 - 2sin^2(x)

推导过程：

我们知道余弦函数（cosx）的基本性质：

- cos(π/2 + x) = sin(x)

- cos(π/2 - x) = -sin(x)

- cos(x) = √[1 + (sin(x))^2]

现在，我们来推导cos(2x)的表达式。

1. 使用余弦的和差化积公式：

- cos(a + b) = cos(a)cos(b) - sin(a)sin(b)

- cos(a - b) = cos(a)cos(b) + sin(a)sin(b)

2. 应用到cos(2x)：

- cos(2x) = cos(π/2 + x)cos(x) - sin(π/2 + x)sin(x)

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

3. 展开并简化：

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin(π/2 + x))

- cos(2x) = cos(x)(cos(π/2 + x) - sin(π/2 + x)) - sin(x)(cos(π/2 + x) - sin