cmr 4 ^20+8^13⋮ 6
1) 15 - 98 + 91 - 75 + 98 - 91 - 13
2) 5 - ( 4 - 7 + 12 ) + ( 4 - 7 + 12 ) - 10
3) 20 - ( 25 - 98 + 12 ) + ( 25 - 98 - 72 )
4) - ( 17 - 4 + 13 + | -17 | + 13 )
5) | -16 | - ( 5 - 8 + 7) - ( - 5 ) - 8
6) -20 + ( 3 - 16 + 7 ) - ( 3 - 16 + 7 )
7) 32 - ( 5 - 8 + 32 ) + ( 5 - 8 + 6 )
8) -20 - ( 25 - 11 + 8 ) + ( 25 - 8 + 20 )
1)-73
2)-5
3)-64
4)-56
5)9
6)-20
7)6
8)-2
Tick nha
đăng tùng câu một thui bạn
bạn làm thế chắc tụi mk xỉu gần hết
1 - 1/2 + 1/3 - 1/4 + 1/5 - 1/6 + ...+ 1/19 - 1/20
= ( 1 + 1/3 + 1/5 + ...+ 1/19 ) - ( 1/2 + 1/4 + ...+ 1/20 )
= ( 1 + 1/2 + 1/3 + 1/4 + ...+ 1/19 + 1/20 ) - 2 . ( 1/2 + 1/4 + ...+ 1/20 )
= ( 1 + 1/2 + 1/3 + ...+ 1/20 ) - ( 1 + 1/2 + ... + 1/10 )
= 1/11 + 1/12 + 1/13 + ...+ 1/20 ( Đpcm )
TK mk nha !!!
\(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{19}-\frac{1}{20}\)
\(=\left(1+\frac{1}{3}+...+\frac{1}{19}\right)-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{20}\right)\)
\(=1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{19}+\frac{1}{20}-2\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{20}\right)\)
\(=1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{20}-1+\frac{1}{2}+....+\frac{1}{10}\)
\(=\frac{1}{11}+\frac{1}{12}+...+\frac{1}{20}\left(đpcm\right)\)
\(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}+.....+\frac{1}{19}-\frac{1}{20}\)
= \(\left(1+\frac{1}{3}+\frac{1}{5}+.........+\frac{1}{19}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+.........+\frac{1}{20}\right)\)
= \(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+......+\frac{1}{19}+\frac{1}{20}-2\left(\frac{1}{2}+\frac{1}{4}+.......+\frac{1}{20}\right)\)
= \(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+..........+\frac{1}{19}+\frac{1}{20}+1+\frac{1}{2}+.............+\frac{1}{20}\)
= \(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+.........+\frac{1}{20}\)
Vậy biểu thức \(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}+.........+\frac{1}{19}-\frac{1}{20}=\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+.......+\frac{1}{20}\)( đpcm)
bài 1:tính số hạng có trong các tổng sau:
1.1-2+3-4+5-6+7-8
2.1-3+5-7+9-11+13-15
3.-1+3-5+7-9+11-13+15
4.2-4+6-8+10-12+14-16+18-20
5.-2+4-6+8-10+12-14+16-18+20
6.2-5+8-11+14-17+20-23+26
7.-1+5-9+13-17+21-25+29-33+37
8.1-5+9-....-81+85-89+93
9.2-4+6-8+....+202-204
10.1-6+11-16+21-....+176-181
so sánh
a)1/2+1/6+1/12+1/20+1/30+1/42 với 60006/70707
b)1/11+1/12+1/13+1/14+.....+1/19+1/20 với 1
c)7/9+8+13 với 8/9+7/13
d)5/6+6/7+7/8+8/5 với 4
a, 20 - ( -40 ) + 10 b, -21 + ( -13 ) - 4 c, ( 3 - 23 ) - ( 4 - 10 ) d, ( -8 + 15 ) - ( -6 - 20 )
a) 20- ( -40 ) +10 b) - 21+ ( -13 ) -4
= ( 40+ 20 ) +10 = - ( 21+13 )- 4
= 60+ 10 = ( -34 ) - 4
= 70 = - ( 34+ 4 )
= - 38
c) ( 3-23 ) - ( 4- 10 ) d) ( -8 + 15 ) - ( -6-20)
= ( - 20 ) - ( -6 ) = 7+ - 26
= - ( 20+ 6 ) = -19
= - 26
b: =-21-13-4=-34-4=-38
d: =-8+15+6+20=7+6+20=33
Tính nhanh :
a, 3, 63 + 5, 13 + 6 , 63 + 8, 13 + 9, 63 + 11 , 13
b, 135 , 7 + 20 , 4 + 4 , 6
a. 3, 63 + 5, 13 + 6 , 63 + 8, 13 + 9, 63 + 11 , 13
= 8,76 + 6 , 63 + 8, 13 + 9, 63 + 11 , 13
= 15,39 + 8, 13 + 9, 63 + 11 , 13
= 23,52 + 9, 63 + 11 , 13
= 33,15 + 11 , 13
= 44,28
b. 135 , 7 + 20 , 4 + 4 , 6
= 156,1 + 4,6
= 160,7
a) 3,63 + 5,13 + 6,63 + 8,13 + 9,63 + 11,13
=(3,63 + 11 ,13) + ( 5,13 + 9,63 ) + (6,63 +8,13)
= 14,76 + 14,76 + 14,76
= 14 , 76 x 3
= 44,28
b) 135,7 + 20,4 + 4,6
= 135,7 + (20,4 + 4,6)
= 135,7 + 25
=160,7
a. ( 19 5/8 : 7/12 - 13 1/4 : 7/12) . 4/5
b.( - 2/5 + 3/7) - (4/9 + 12/20 - 13/35) + 7/35
c. ( 4 5/57 - 3/ 4/51 + 8 13/29) - ( 3 5/57 - 6 16/29)
\(\left(19\frac{5}{8}:\frac{7}{12}-13\frac{1}{4}:\frac{7}{12}\right).\frac{4}{5}\)
\(=\left(\frac{157}{8}:\frac{7}{12}-\frac{53}{4}:\frac{7}{12}\right).\frac{4}{5}\)
\(=\left[\left(\frac{157}{8}-\frac{53}{4}\right):\frac{7}{12}\right].\frac{4}{5}\)
\(=\left[\frac{51}{8}:\frac{7}{12}\right].\frac{4}{5}\)
\(=\frac{153}{14}.\frac{4}{5}\)
\(=\frac{306}{35}\)
\(\left(\frac{-2}{5}+\frac{3}{7}\right)-\left(\frac{4}{9}+\frac{12}{20}-\frac{13}{35}\right)+\frac{7}{35}\)
\(=\frac{1}{35}-\frac{212}{315}+\frac{7}{35}\)
\(=\frac{1}{35}+\frac{-212}{315}+\frac{7}{35}\)
\(=\frac{9}{315}+\frac{-212}{315}+\frac{63}{315}\)
\(=\frac{-140}{315}=\frac{-4}{9}\)
\(\left(4\frac{5}{57}-3\frac{4}{51}+8\frac{13}{29}\right)-\left(3\frac{5}{57}-6\frac{16}{29}\right)\)
\(=\left(\frac{233}{57}-\frac{157}{51}+\frac{245}{29}\right)-\left(\frac{176}{57}-\frac{190}{29}\right)\)
\(=\frac{1941}{323}-\frac{-5726}{1653}\)
\(=\frac{14011}{1479}\)
a, 20-(-40)+10
b, -21+(-13)-4
c, (3-23)-(4-10)
d, (-8+10)-(-6-20)
20 - ( - 40 ) + 10 ....................
= 20 + 40 + 10..............
= 60 + 10 = 70...........
a, 20-(-40)+10
=60+10=70
b, -21+(-13)-4
=-34-4=-38
c, (3-23)-(4-10)
=-20-(-6)=-14
d, (-8+10)-(-6-20)
=2-(-26)=28
-21 + ( - 13 ) - 4.....................
= - 21 - 13 - 4 ........................
= -21 - 17 = - 38.....................
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+19+20/20
= 1 + 2 + 3 + .... + 18 + 19 + 1
= ( 1 + 19 ) + ( 2 + 18 ) + ( 3 + 17 ) +.... + ( 10 + 1 )
= 20 + 20 + 20 +... + 11
= (20 × 9) + 11
= 191