`#DyHungg`
`a) 1/5+(-1)/6+1/7+(-1)/8+1/9+1/8+(-1)/7+1/6+(-1)/5`
`=(1-1)/5+(-1+1)/6+(1-1)7+(1-1)8+1/9`
`=0+0+0+0+1/9`
`=1/9`
...................
`b) -7/21+(1+1/3)`
`=-7/21+1/3+1`
`=-7/21+7/21+1`
`=0+1`
`=1`
........................
`c) 4/20+16/42+6/15+(-3)/5+2/21+(-10)/21+3/20`
`=(4/20+6/15-3/5)+(16/42+4/42-20/42)+3/20`
`=3/20`
