已知信号<img src="https://up.zaixiankaoshi.com/docs/41/34/4134ac749b386df3f8d48030dff623e6.png">的傅里叶变换为<img src="https://up.zaixiankaoshi.com/docs/fa/9d/fa9d4b02e8addda4037bc0aede5a922b.png">，则<img src="https://up.zaixiankaoshi.com/docs/10/c6/10c6f20234a815ea1335057a58e8a727.png">为（ ）