expression前面加a还是an

前面用an，例如：

1、Well， now we have an expression for the energy.好，现在我们有能量的表达式。

2、Its key is to find an expression for coefficient β.关键在于β系数表达式的求出。

3、He screwed his face into an expression of mock pain.他面部扭曲，假装疼痛。