图示交变应力的平均应力<img src="https://static.zxtiku.com/wangke/imgs/topic/64f2ad8e7e68acc571956bce62f2684d.png"/>和应力幅<img src="https://static.zxtiku.com/wangke/imgs/topic/75350baf06832129198e3e47f1c3c8c4.png"/>分别为（）<br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/e470f3d6ad3a6b1fe5f8a2a501b4dd06.jpg"/>