1. No one helped Jack. He worked out the math problem by ________ (他). 2. ________ (浪费) time means killing yourself. 3. I didn"t _________ (认识到) that I was wrong until he told me. 4. We can _________ (容易) become unhappy unless we deal with our problems. 5. We"ll have a football match on _______ (星期五) afternoon.