Tính bằng cách hợp lý
B=(5/2019+4/2020-3/2021).(1/2-1/3-1/6)
tính bằng cách thuận tiện
1-2 + 3-4 + 5-6 + 7-8 + ....... + 2019-2020 + 2021
Tính hợp lý:
a) A=\(2021^2\)-2020.(2021+1)
b) B=1+2-3-4+5+6-7-8+9+10-...+2018-2019-2020+2021
c) C=\(\dfrac{5.4^{15}.9^9-4.3^{20}.8^9}{5.2^9.6^{19}-7.2^{29}.27^6}\)
a)= 2021.2021-2020.(2021+1)
= 2021.(2020+1)-2020.(2021+1)
= (2021.2020)+2021-(2020.2021)-2020
= 1
b) B= (1+2-3-4)+(5+6-7-8)+(9+10-11-12)...........+(2017+2018-2019-2020)+2021
B= -4+(-4)+....................(-4)+2021
B= -4x505+2021
B= -2020 + 2021
B = 1
Tính A=1/2×3×4+1/3×4×5+1/4×5×6+...+1/2019×2020×2021
Tính hợp lí (nếu có thể):
A= -1-2+3+4-5-6+7+8-9-10+...+2019+2020-2021-2022+2023
Lời giải:
$A=(-1-2+3+4)+(-5-6+7+8)+(-9-10+11+12)+...+(-2021-2022+2023+2024)-2024$
$=\underbrace{4+4+...+4}_{506}-2024$
$=4.506-2024=0$
tính nhanh: (1+3+5+7+...+2019+2021)-(2+4+6+8+...+2020)
(1+3+5+7+...+2019+2021)
A=1−3+5−7+......−2019+2021−2023
A=(1−3)+(5−7)+....+(2021−2023)A=(1−3)+(5−7)+....+(2021−2023)
A=−2+(−2)+....+(−2)(506)A=−2+(−2)+....+(−2)(506cặp)
a=−2.506A=−2.506
A=−1012A=−1012
(2+4+6+8+...+2020)
B=2+4+6+8+...+2018+2020
B = 2(1 + 2 + 3 + 4 + ... + 1009 + 1010)
B = 2 . (1011 . 1010 : 2)
B = 2 . 510555
B = 1 021 110
(1+3+5+7+......+2019+2021)-(2+4+6+8+.....+2020)
\(=\dfrac{\left(1+2021\right).\left[\left(2021-1\right):2+1\right]}{2}-\dfrac{\left(2+2020\right).\left[\left(2020-2\right):2+1\right]}{2}\)
\(=1011\)
Tính S= 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + 9 + 10 - ... + 2018 - 2019 - 2020 + 2021
S=1+(2-3)+(-4+5)+(6-7)+(-8+9)+...+(-2020+2021)
S=1-1+1-1+1+...+1
S=1+0+0+...+0
S=1
\(S=1+2-3-4+...+2017+2018-2019-2020+2021\\ S=\left(1+2-3-4\right)+...+\left(2017+2018-2019-2020\right)+2021\\ S=\left(-4\right)+\left(-4\right)+\left(-4\right)+...+-4+2021\\ S=505.\left(-4\right)+2021\\ S=-2020+2021\\ S=1\)
Tính S= 1+2-3-4+5+6-7-8+9+10- .......+2018-2019-2020+2021
tính :
A= 1+2-3-4+5+6-7-8+9+...+2018-2019-2020+2021-2022
A = 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + ... + 2018 – 2019 - 2020 + 2021
A = (1 + 2 - 3 - 4) + ... + (2017 + 2018 – 2019 - 2020) + 2021
A = (-4) + ... + (-4) + 2021 +
2020 : 4 = 505
A = (-4) . 505 + 2021
A = (-2020) + 2021
A = 1
Vậy A=1
Mình gửi bạn nha !!!!!