1,
A, = 135+360+65+40
= (135+65) + (360+40)
= 200+400
= 600
B, 463+318+137+22
= (463+137) + (318+22)
= 600+340
= 940
C, 20+21+22+...+30
= (20 + 30) + (21 + 29) + (22 + 28) + (23 + 27) + (24 + 26) + 25
= 50 + 50 + 50 + 50 + 50 + 25
= 5.50 + 25
= 250 + 25
= 275
2,
A, 46+17+54
= (46+54)+17
= 100+17
= 171
B, 4*37*25
= (4*25)*37
= 100*37
= 3700
C, 87*36+64*87
= 87*(36+64)
= 87*100
= 8700
3, $\frac{1}{9.10}+\frac{1}{10.11}+\frac{1}{11.12}+...+\frac{1}{98.99}$
= $\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{98}- \frac{1}{99}$
= $\frac{1}{9}-\frac{1}{99}$
= $\frac{11}{99}-\frac{1}{99}$
= $\frac{10}{99}$
4, $\frac{25}{24}.\frac{36}{35}.\frac{49}{48}.\frac{64}{63}.\frac{81}{80}.\frac{100}{99}.\frac{121}{120}$
= $\frac{5.5}{4.6}.\frac{6.6}{5.7}.\frac{7.7}{6.8}.\frac{8.8}{7.9}.\frac{9.9}{8.10}.\frac{10.10}{9.11}.\frac{11.11}{10.12}$
= $\frac{5.11}{12}$
= $\frac{55}{12}$
Sorry mk tl hơi chậm.
