play an instrument还是play the instrument？

是play the instrument，例如：

1.She learned to play the instrument when she was a small child and gave her first public recital at age 9.她在幼年时就学会了钢琴演奏，9岁就第一次登台演出。

2.The moon was bright and a cool breeze was blowing gently. With myriads9 of thoughts welling up in his mind, he began to play the musical instrument.面对清风明月，他思绪万千，于是又弹起琴来，琴声悠扬，渐入佳境。