Đáp án:
a, S=`5/(3.5)+5/(5.7) +5/(7.9) +.................+ 5/(61.63)`
=`5/2 . ( 2/(3.5) +2/(5.7) +2/(7.9) +........... +2/(61.63) )`
=`5/2 . (1/3-1/5+1/5-1/7+ ............+1/61-1/63)`
=`5/2 . (1/3-1/63)`
=`5/2. 20/63`
=`50/63`
b, Ta có : `7/15< 7/14=1/2`
`20/39>20/40=1/2`
`⇒20/39>1/2>7/15`
HAY `20/39>7/15`
XIN HAY NHẤT NHA
@hoang
