Giải thích các bước giải:
a) `(-1)/20+(-1)/30+(-1)/42+(-1)/56+(-1)/72+(-1)/90`
`=-(1/20+1/30+1/42+1/56+1/72+1/90)`
`=-(1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10)`
`=-(1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)`
`=-(1/4-1/10)`
`=-3/20`
b) Ta có:
`2010/2011>2010/(2011+2012+2013)`
`2011/2012>2011/(2011+2012+2013)`
`2012/2013>2012/(2011+2012+2013)`
`2010/2011+2011/2012+2012/2013>2010/(2011+2012+2013)+2011/(2011+2012+2013)+2012/(2011+2012+2013)`
`=>2010/2011+2011/2012+2012/2013>(2010+2011+2012)/(2011+2012+2013)`
`=>P>Q`
Vậy `P>Q.`