题目

My father sold everything we owned, took all the money and disappeared from our lives. My mother suddenly found herself 21 to care for five boys. I worried about the situation, but my mother stayed 22 , and assured us that she would keep us together as a 23 and safe from harm. Although small in size, she could 24 heavy horse-drawn plows. Besides, she 25 found time to help us with homework. As Christmas approached, my mother didn’t seem to smile as much. I 26 she had no money to buy us presents. Considering myself practically grown, I hid my 27 . One day, my mother took a saw(锯)into 13．证明：$\frac{1-ta{n}^{2}x}{1+ta{n}^{2}x}$=cos2x-sin2x．