= 1/( 1*3)+ 1/(3*5)+ 1/(5*7)+...+ 1/( 15* 17)

= 1/2*( 1- 1/3)+ 1/2*( 1/3- 1/5)+...+ 1/2*( 1/ 15- 1/ 17)

= 1/2*( 1- 1/3+ 1/3- 1/5+...+ 1/ 15- 1/ 17)

= 1/2*( 1- 1/ 17)

=8/ 17

Sequence problem 1/9, 1, 7, 36, (d)

A.74 B.86 C.98 D. 125

1/9 is 9-1 power. 1 is the power of 8, 7 is the power of 7, 36 is the power of 6, followed by 5, so it is 125.

The 20% discount is because the original price is ×( 1+20%)×2÷3=4/5=0.8.

Spending 20% price increase in 400 yuan to buy goods in 600 yuan is equivalent to buying goods from 500 yuan before the price increase, that is, 400 divided by 500 is the discount to enjoy, 20% discount.