`#Natsu`
a) Cho `6A= 1.3.6+3.5.(7-1)+5.7.(9-3)+...+97.99.(101-95)`
`6A= 1.3.6+3.5.7-1.3.5+5.7.9-3.5.7+...+97.99.101-95.97.99`
Loại ra những phân số giống nhau ta còn:
`6A=1.3.6-1.3.5+97.99.101`
`6A=969906`
`A=969906 : 6`
`A= 161651`
b ) Đặt `A=1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100`
`4A=(1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100)4`
`4A=1.2.3(4-0)+2.3.4(5-1)+3.4.5(6-2)+4.5.6(7-3)+...+98.99.100(101-97)`
`4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+4.5.6.7-3.4.5.6+...+98.99.100.101-97.98.99.100`
`4A=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+3.4.5.6-3.4.5.6+...+97.98.99.100-97.98.99.100+98.99.100.101`
`4A=98.99.100.101`
`=>A=98.99.100.101/4`
`A=24497550`
