二次方差公式:a² - b² = (a+b)(a-b)
完全平方公式:a² + 2ab + b² = (a+b)²
正弦函数:sin(x+y) = sin(x)cos(y) + cos(x)sin(y)
余弦函数:cos(x+y) = cos(x)cos(y) - sin(x)sin(y)
正切函数:tan(x+y) = (tan(x) + tan(y))/(1 - tan(x)tan(y))