15．口算．
| $\frac{4}{15}+\frac{7}{15}$= | $\frac{9}{11}-\frac{7}{11}$= | $\frac{5}{12}+\frac{7}{12}$= | $\frac{5}{6}-\frac{1}{6}$= |
| $\frac{9}{19}+\frac{4}{19}$= | $\frac{9}{16}-\frac{3}{16}$= | $\frac{1}{3}-\frac{1}{7}$= | $\frac{1}{6}-\frac{1}{7}$= |
| $\frac{1}{5}-\frac{1}{8}$= | $\frac{1}{8}+\frac{1}{9}$= | $\frac{1}{2}+\frac{1}{5}$= | $\frac{1}{4}+\frac{1}{5}$= |
11．求出未知角的度数（表中∠1、∠2、∠3是三角形的三个内角）
0 160044 160052 160058 160062 160068 160070 160074 160080 160082 160088 160094 160098 160100 160104 160110 160112 160118 160122 160124 160128 160130 160134 160136 160138 160139 160140 160142 160143 160144 160146 160148 160152 160154 160158 160160 160164 160170 160172 160178 160182 160184 160188 160194 160200 160202 160208 160212 160214 160220 160224 160230 160238 199269
| ∠1 | 75° | 90° | 60° | 50° | |
| ∠2 | 65° | 120° | 60° | ||
| ∠3 | 42° | 15° | 50° |