1. Tính nhanh
a) A=1/ 5 .6 + 1/ 6 .7 + 1/ 7. 8+ ... + 1/24 . 25
b) B= 2/ 3 .5 + 11/ 8 .19 + 13/ 19 .32 + 25/ 32 . 57 + 30/ 57 . 87
1. Tính nhanh
a) A=1/ 5 .6 + 1/ 6 .7 + 1/ 7. 8+ ... + 1/24 . 25
b) B= 2/ 3 .5 + 11/ 8 .19 + 13/ 19 .32 + 25/ 32 . 57 + 30/ 57 . 87
a: \(A=\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+...+\dfrac{1}{24}-\dfrac{1}{25}\)
\(=\dfrac{1}{5}-\dfrac{1}{25}=\dfrac{4}{25}\)
b: \(B=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{8}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{32}+\dfrac{1}{32}-\dfrac{1}{57}+\dfrac{1}{57}-\dfrac{1}{87}\)
\(=\dfrac{2}{15}+\dfrac{1}{8}-\dfrac{1}{87}\)
=859/3480
(/ là phần, * là cộng)
Tính nhanh:
A= 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72
B= 1/5*6 + 1/6*7 + 1/7*8 +........+ 1/24*25
C= 2/3*5 + 3/5*8 + 11/8*19 + 13/19*32 + 25/32*57 + 30/57*87
A = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72
= 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9
= 1- 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9
= 1 - 1/9 = 8/9
Câu B, C dấu * là nhân hay công vậy?
tính A=2/3*5+3/5*8+11/8*19+13/8*19+25/32*57+30/57*87
\(A=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{19}+...+\dfrac{1}{57}-\dfrac{1}{87}=\dfrac{1}{3}-\dfrac{1}{87}=\dfrac{28}{87}\)
2/3•5+3/5•8+11/5•19+13/19•33+25/32•57+30/57•87
Sửa đề:
\(\dfrac{2}{3.5}+\dfrac{3}{5.8}+\dfrac{11}{8.19}+\dfrac{13}{19.32}+\dfrac{25}{32.57}+\dfrac{30}{57.87}\)
\(=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{32}+\dfrac{1}{32}-\dfrac{1}{57}+\dfrac{1}{57}-\dfrac{1}{87}\)
(do \(\dfrac{n}{a.\left(a+n\right)}=\dfrac{1}{a}-\dfrac{1}{a+n}\) với \(a\in N\)*)
\(=\dfrac{1}{3}-\dfrac{1}{87}=\dfrac{28}{87}\)
Chúc bạn học tốt!!!
Câu 1: (1đ) Sắp xếp các số nguyên theo thứ tự tăng dần: (-1)4; -38; -45; |-15|; 0; (-2)3; 13; 29; 33; -
Câu 2. (2đ) Tính hợp lí. a) -30.6 - 10. (12 - 18)
b) 25.(32 + 47) - 32(25 + 47)
c) 23.(-57) + 57.(-87) – 20 . 57
d) 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + ....- 99 - 100 +101 + 102 -103 -104 +105 + 106
Câu 3: (3đ) Tìm số nguyên x, biết: a) x – (-2) = 32 + (-19)
c) 2|x + 8| = -4 + 6
b) 2x – 30 = x + 49
Câu 4: (1đ) a) Tìm x, y biết xy + 3x – 7y = 21. b) Tìm GTNN của A = çx - 5ç + 24.
Tính nhanh nếu có thể: b)B=5/2 + 6/-11 + 3/8 - 7/2 + 6/8 + -5/11 c)C=4/3 + 3/5 +7/3 + 2/5 - -1/3 d)D=-5/6 . 4/19 + -7/12 . 4/19 - 40/57 e)E=1/3 . 4/5 + 1/-3 . 9/5 + 2/3
b: \(B=\dfrac{5}{2}-\dfrac{7}{2}+\dfrac{3}{8}+\dfrac{6}{8}+\dfrac{-6}{11}-\dfrac{5}{11}=-2-1+\dfrac{9}{8}=\dfrac{9}{8}-3=-\dfrac{15}{8}\)
c: \(C=\left(\dfrac{4}{3}+\dfrac{7}{3}+\dfrac{1}{3}\right)+\left(\dfrac{2}{5}+\dfrac{3}{5}\right)=4+1=5\)
d: \(D=\dfrac{4}{19}\left(\dfrac{-5}{6}-\dfrac{7}{12}\right)-\dfrac{40}{57}\)
\(=\dfrac{4}{19}\cdot\dfrac{-17}{12}-\dfrac{40}{57}=-1\)
e: \(E=\dfrac{1}{3}\left(\dfrac{4}{5}-\dfrac{9}{5}\right)+\dfrac{2}{3}=\dfrac{2}{3}-\dfrac{1}{3}=\dfrac{1}{3}\)
Tính nhanh
a) 31/23 - (7/32 + 8/23)
b) 38/45 - (8/45 - 17/51 - 3/11)
c)(1/3 + 12/67 + 13/41) - ( 79/67 - 28/41)
d) 1/5 + -1/6 + 1/7 + -1/8 + 1/9 + 1/8 + -1/7 + 1/6 + -1/5
Tính nhanh
a) 31/23 - (7/32 + 8/23)
b) 38/45 - (8/45 - 17/51 - 3/11)
c)(1/3 + 12/67 + 13/41) - ( 79/67 - 28/41)
d) 1/5 + -1/6 + 1/7 + -1/8 + 1/9 + 1/8 + -1/7 + 1/6 + -1/5
`a,`
`31/23-(7/32+8/23)`
`=31/23-7/32-8/23`
`=(31/23-8/23)-7/32`
`=1-7/32=25/32`
`b,`
`38/45-(8/45-17/51-3/11)`
`=38/45-8/45+17/51+3/11`
`= (38/45-8/45)+17/51+3/11`
`=2/3+17/51+3/11`
`=1+3/11=14/11`
`c,`
`(1/3+12/67+13/41)-(79/67-28/41)`
`= 1/3+12/67+13/41-79/67+28/41`
`= 1/3+(12/67-79/67)+(13/41+28/41)`
`= 1/3+(-1)+1=1/3+(-1+1)=1/3+0=1/3`
`d,`
`1/5+(-1/6)+1/7+(-1/8)+1/9+1/8+(-1/7)+1/6+(-1/5)`
`= (1/5+ -1/5)+(-1/6+1/6)+(1/7+ -1/7)+(-1/8 +1/8)+1/9`
`= 0+0+0+0+1/9=1/9 .`