2/5x7+2/5x9+2/9×11+2/11×13+2/13×15
=$\frac{1}{5}$ -$\frac{1}{7}$ +$\frac{1}{7}$ -$\frac{1}{9}$ +$\frac{1}{9}$ -$\frac{1}{11}$ +$\frac{1}{11}$ -$\frac{1}{13}$ +$\frac{1}{13}$ -$\frac{1}{15}$
=$\frac{1}{5}$ -$\frac{1}{15}$
=$\frac{3}{15}$ -$\frac{1}{15}$
=$\frac{2}{15}$
