\(B=\frac{1}{1\cdot2\cdot3}+\frac{1}{2\cdot3\cdot4}+\frac{1}{3\cdot4\cdot5}+\frac{1}{18\cdot19\cdot20}\)

\(B=\frac{1}{2}\left(\frac{2}{1\cdot2\cdot3}+\frac{2}{2\cdot3\cdot4}+\frac{2}{3\cdot4\cdot5}+\frac{2}{18\cdot19\cdot20}\right)\)

\(B=\frac{1}{2}\left(\frac{1}{1\cdot2}-\frac{1}{2\cdot3}+\frac{1}{2\cdot3}-\frac{1}{3\cdot4}+\frac{1}{3\cdot4}-\frac{1}{4\cdot5}+...+\frac{1}{18\cdot19}-\frac{1}{19\cdot20}\right)\)

\(B=\frac{1}{2}\left(\frac{1}{1\cdot2}-\frac{1}{19\cdot20}\right)\)

\(B=\frac{1}{2}\cdot\frac{189}{380}=\frac{189}{760}\)

\(C=\frac{52}{1\cdot6}+\frac{52}{6\cdot11}+\frac{52}{11\cdot16}+...+\frac{52}{31\cdot36}\)

\(C=\frac{52}{5}\left(\frac{5}{1\cdot6}+\frac{5}{6\cdot11}+\frac{5}{11\cdot16}+...+\frac{6}{31\cdot36}\right)\)

\(C=\frac{52}{5}\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+...+\frac{1}{31}-\frac{1}{36}\right)\)

\(C=\frac{52}{5}\cdot\left(1-\frac{1}{36}\right)\)

\(C=\frac{91}{9}\)

\(=\dfrac{1}{2}\cdot\left(\dfrac{2}{1\cdot2\cdot3}+\dfrac{2}{2\cdot3\cdot4}+...+\dfrac{2}{50\cdot51\cdot52}\right)\)

\(=\dfrac{1}{2}\left(\dfrac{1}{1\cdot2}-\dfrac{1}{2\cdot3}+\dfrac{1}{2\cdot3}-\dfrac{1}{3\cdot4}+...+\dfrac{1}{50\cdot51}-\dfrac{1}{51\cdot52}\right)\)

\(=\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{51\cdot52}\right)\)

\(=\dfrac{1}{2}\cdot\dfrac{1325}{2652}=\dfrac{1325}{5304}\)

1: =7/2+23/5=35/10+46/10=81/10

2: =6/8+53/8=59/8

3: =43/8+65/7=821/56

\(a,3\dfrac{1}{2}+4\dfrac{3}{5}=\dfrac{81}{10}\\ b,\dfrac{3}{4}+6\dfrac{5}{8}=\dfrac{59}{8}\\ 5\dfrac{3}{8}+9\dfrac{2}{7}=\dfrac{821}{56}\\ 4,4\dfrac{1}{2}-2\dfrac{3}{10}=\dfrac{11}{5}\)

\(\text{C. (-3) –(4- 6) = -1}\)

;C

C

a) 5 + (-8) . 3 = 5 + (-24) = -19

b) 4 +  - 5 2  = 4 + 25 = 29

c) 1 – 2 – 3 + 4 + 5 – 6 – 7 + 8 + … + 801 – 802 – 803 + 804

= (1 – 2 – 3 + 4) + (5 – 6 – 7 + 8) + … + (801 – 802 – 803 + 804)

= 0 + 0 + … + 0 = 0

Câu 1: D

Câu 3: Các kết quả sai là B và C

Câu 4: A

1 + 6 + 5 + 4 + 7 + 8 + 9 + 5 + 5 + 6 +52 + 2 + 5 + 5 = 120

ai nhanh nhất minh k 

=120.mk nhanh nhất.k nha