你把分子分母写倒了.二分之一应该写为：1/2
所以,你的式子应该是：
1/2 + 1/6 + 1/12 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90
= 1/(1 * 2) + 1/(2 * 3) + 1/(3 * 4) + 1/(4 * 5) + 1/(5 * 6) + 1/(6 * 7 + 1/(7 * 8) + 1/(8 * 9) + 1/(9 * 10)
= (1 - 1/2) + (1/2 - 1/3) + (1/3 - 1/4) + (1/4 - 1/5) + (1/5 - 1/6) + (1/6 - 1/7) + (1/7 - 1/8) + (1/8 - 1/9) + (1/9 - 1/10)
= 1 - 1/10
= 9/10