Đáp án:
Bài 2:
Ta có: A=3/2X4 + 3/4X6 + 3/6X8 + 3/8X10 +......+ 3/98X100
A/3 =1/2X4 + 1/4X6 + 1/6X8 + 1/8X10 +......+ /98X100
2A/3=2/2X4 + 2/4X6 + 2/6X8 + 2/8X10 +......+ 2/98X100
2A/3=1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+......+1/98-1/100
2A/3=1/2-1/100=50/100-1/100=49/100
A/3=49/100X1/2=49/200
A=49/200X3=147/200<1
Vậy: A<1
Bài 3:
Ta có :
1/6<1/5
1/7<1/5
.................
1/9<1/5
1/10=1/10
1/11<1/10
..................
1/17<1/10
⇔1/5+1/6+1/7+......+1/17<1/5+1/5+....+1/5⇔1/5+1/6+1/7+......+1/17<1/5+1/5+....+1/5
⇔A<1/5.5+1/10.8⇔A<1/5.5+1/10.8
⇔A<1+4/5=9/5<2⇔A<1+4/5=9/5<2
⇔A<2(đpcm)
