expression前面加a還是an

前面用an，例如：

1、Well， now we have an expression for the energy.好，現在我們有能量的表達式。

2、Its key is to find an expression for coefficient β.關鍵在於β係數表達式的求出。

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