`3)`
`a) A= 1.2 + 2.3+ 3.4 +…+ 100.101`
`3A = 1.2.3+2.3.3+3.4.3+...+100.101.3`
`3A = 1.2.3+2.3.(4-1)+3.4.(5-2)+...+100.101.(102-99)`
`3A= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+100.101.102-99.100.101`
`3A= (1.2.3-1.2.3)+...+(99.100.101-99.100.101)+100.101.102`
`3A= 100.101.102`
`A = 100.101.102 : 3`
`A = 343 400`
`b) B= 1.2.3 + 2.3.4 + 3.4.5 +…+ 100.101.102`
`4B=1.2.3.4+2.3.4.4+3.4.5.4+...+100.101.102.4`
`4B=1.2.3.4+2.3.4.(5-1)+3.4.5.(6-2)+...+100.101.102.(103-99)`
`4B=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...+100.101.102.103-99.100.101.102`
`4B=(1.2.3.4-1.2.3.4)+...+(99.100.101.102)+100.101.102.103`
`4B=100.101.102.103`
`B= 100.101.102.103:3`
`B= 35370200`
`4)`
`a) A= (714+286)+(382+318)`
`A=1000+700`
`A=1700`
`b) B=35.34+35.86+65.75+65.45`
`B=35.(34.86)+65.(75.45)`
`B=35.120+65.120`
`B=120.(35+65)`
`B=120.100`
`B=12000`
`c)C= (257.139 – 257 .39):100`
`C=257.(139-39):100`
`C=257.(100:100)`
`C=257`
`d)D=102 – 96 + 90 - 84 +…+ 54 – 48`
`D=(102-96)+(90-84)+(78-72)+(66-60)+(54-48)`
`D=6.5`
`D=30`
`e) E=3+5+7+...+2021`
`E=[(2021-3):3+1]`(số số hạng)`=678` số
`=> [(3+2021).678:2`
`= 686136`