Đặt A = 1 + ( - 2 ) + 3 + ( - 4 ) + .... + 19 + ( - 20 )
⇒ A = [ 1 + ( - 2 ) ] + [ 3 + ( - 4 ) ] + .... + [ 19 + ( - 20 ) ]
⇒ A = ( - 1 ) + ( - 1 ) + ... + ( - 1 ) [ có 10 số ( - 1 ) ]
⇒ A = ( - 1 ) . 10
⇒ A = - 10
Vậy A = - 10
2 tính
1 + ( -2) + 3 + ( -4) +....+ 19 + ( - 20)
cíu lần 2
Tính
1. (-8537) + (1975 + 8537)
2.(35 - 17) - (-17 + 20 + 35)
Bài 1: Thực hiện phép tính
1) 17 . 85 + 15 . 17 – 120
| 9)
|
2) 23 . 17 – 23 . 14 | 10) |
3) 20 + | 11) 5871: {928 -[(-82) +247) ].5} |
4) 28.76+23.28 -28.13 | 12) 112:11-55 |
5) 15.36 – 15.10 + 26.85 | 13) |
6) 75 – ( 3.52 – 4.23) | 14) |
7) 4.52 – 3.23 + 33:32 | 15) |
8) 80 – (4 . 52 – 3 . 23) | 9) 80 – 73.3 10) -8 – 51.2 |
Bài 1. Thực hiện phép tính
1) 
2) 
3) 
4) 
5) 
6) 
7) 
35:7
Tính
1 - 1/2;1/2 - 1/3; 1/3 - 1/4; 1/4 - 1/5; 1/5 - 1/6; 1/6 - 1/7
4)cho a^1+a^2+a^3+...+a^19+a^20+a^21=10 va a^1+a^2=a^3+a^4=...=a^19+a^20=a^20+a^21=2. tim a^20
E=1×2×3+2×3×4×3×4×5+...+18×19×20
G=2×4×6+4×6×8+6×8×10+...-18×20×22
H=1×3×5+3×5×7+5×7×9+17×19×21
A=2^19*27^3+15*64^3*(-9)^4/2^19*3^9+(-3)^10*2^20
So sánh 1/2 + 1/3 + 1/4 + ... + 1/18 + 1/19 + 1/20 và 19/20
1)\(\dfrac{1}{2}+\dfrac{13}{19}-\dfrac{4}{9}+\dfrac{6}{19}+\dfrac{5}{18}\)
2)\(\dfrac{ }{\dfrac{-20}{23}+\dfrac{2}{3}-\dfrac{3}{23}+\dfrac{2}{5}+\dfrac{7}{15}}\)
3)\(\dfrac{ }{\dfrac{4}{3}+\dfrac{-11}{31}+\dfrac{3}{10}-\dfrac{20}{31}-\dfrac{2}{5}}\)
4)\(\dfrac{ }{\dfrac{5}{7}.\dfrac{5}{11}+\dfrac{5}{7}.\dfrac{2}{11}-\dfrac{5}{7}.\dfrac{14}{11}}\)
