A=1*2+2*3+3*4+...+2017*2018

3A=1*2*3+2*3*(4-1)+...+2017*2018*(2019-2016)

3A=1*2*3+2*3*4-1*2*3+...+2017*2018*2019-2016*2017*2018

3A=2017*2018*2019

A=\(\frac{2017.2018.2019}{3}\)

mk chỉ biết tính a thôi

thank you 

b: Ta có: \(x=-\dfrac{31}{33}=1-\dfrac{2}{33}\)

\(y=-\dfrac{49}{51}=1-\dfrac{2}{51}\)

mà \(-\dfrac{2}{33}>-\dfrac{2}{51}\)

nên x>y

 15^19 thành 16^19

Ta có:

16^19=(2^4)^19=2^76

8^25=(2^3)^25=2^75

2^76>2^75⇒16^19>8^25

Trả lời:

\(10+11+12+13+14+15+16+17+18+19+20\)

\(=\left(10+20\right)+\left(11+19\right)+\left(12+18\right)+\left(13+17\right)+\left(14+16\right)+15\)

\(=30+30+30+30+30+15\)

\(=165\)

\(1+2+3+4+5+6+7+8+9\)

\(=\left(1+9\right)+\left(2+8\right)+\left(3+7\right)+\left(4+6\right)+5\)

\(=10+10+10+10+5\)

\(=45\)

10+11+12+13+14+15+16+17+18+19+20

(10+20)+(11+19)+(12+18)+(13+17)+(14+16)+15

30+30+30+30+30+15

165

\(a,\text{10 + 11 + 12 + 13 + 14 + 16 + 17 + 18 + 19 + 20}\)

\(=\left(10+20\right)+\left(11+19\right)+\left(12+18\right)+\left(13+17\right)+\left(14+16\right)\)

\(=30+30+30+30+30\)

\(=30\times5\)

\(=150.\)

\(b,\text{1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9}\)

\(=\left(1+9\right)+\left(2+8\right)+\left(3+7\right)+\left(4+6\right)+5\)

\(=10+10+10+10+5\)

\(=45.\)

\(\frac{-1}{6}.\frac{-15}{19}.\frac{38}{45}\)

\(\frac{\left(-1\right).\left(-15\right).2.19}{2.3.19.3.15}\)

\(=\frac{1}{3.3}=\frac{1}{9}\)

\(\left(-\frac{1}{6}\right).\left(-\frac{15}{19}\right).\frac{38}{45}\)

\(=\frac{\left(-1\right).\left(-15\right).38}{6.19.45}=\frac{1.15.38}{6.19.45}\)

\(=\frac{1.1.1}{3.1.3}=\frac{1}{9}\)

\(\frac{-1.\left(-15\right).38}{6.19.45}\)=\(\frac{-1.\left(-3\right).2}{6.9}\)=\(\frac{-1\left(-1\right).2}{2.9}\)=\(\frac{1}{9}\)

Tác dụng : nhân hóa

Nhân hóa???

cả so sánh và nhân hóa

C = 19³⁰+5/19³¹+5

=>19C = 19³¹+95/19³¹+5 = 1+ [90/19³¹+5]

D = 19³¹+5/19³²+5

=>19D = 19³²+95/19³²+5 = 1 + [90/19³²+5]

ma 90/19³¹+5 > 90/19³²+5

=>19C > 19D

=>C > D