Bài 3: Tính tổngA, 1 + (-2) + 3 + (-4) +....+ 19 + (-20)B, 1 – 2 + 3 – 4 +...+99 – 100C, 2 – 4 + 6 – 8 +...+ 46 – 48 + 50 – 52D, - 1 + 3 – 5 + 7 -...-97 + 99E, 1 + (-2) + 3 + (-4) +...+ 1999 + (-2000) + 2001 Bài 4: Tính giá trị biểu thứcA, (2ab2) : (abc) với a = 4, b = -6, c = 12B, [(-25). (-27). (-x)]: y với x = -4, y = -9C, (a2 – b2) : (a + b) (a – b) với a = 5, b =...
Đọc tiếp
Bài 3: Tính tổng
A, 1 + (-2) + 3 + (-4) +....+ 19 + (-20)
B, 1 – 2 + 3 – 4 +...+99 – 100
C, 2 – 4 + 6 – 8 +...+ 46 – 48 + 50 – 52
D, - 1 + 3 – 5 + 7 -...-97 + 99
E, 1 + (-2) + 3 + (-4) +...+ 1999 + (-2000) + 2001
Bài 4: Tính giá trị biểu thức
A, (2ab2) : (abc) với a = 4, b = -6, c = 12
B, [(-25). (-27). (-x)]: y với x = -4, y = -9
C, (a2 – b2) : (a + b) (a – b) với a = 5, b = -3
Bài 3 :
a) \(1+\left(-2\right)+3+\left(-4\right)+...+19+\left(-20\right)\)\(=\left[1+\left(-2\right)\right]+\left[3+\left(-4\right)\right]+...+\left[19+\left(-20\right)\right]\)
\(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)
\(=\left(-1\right)\cdot10=-10\)
b) \(1-2+3-4+...+99-100=\left(1-2\right)+\left(3-4\right)+...+\left(99-100\right)\)
\(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)
\(=\left(-1\right)\cdot50=-50\)
c) \(2-4+6-8+...+46-48+50-52=\left(2-4\right)+\left(6-8\right)+...+\left(50-52\right)\)
\(=\left(-2\right)+\left(-2\right)+...+\left(-2\right)\)
\(=\left(-2\right)\cdot13=-26\)
d) \(-1+3-5+7-...-97+99\)\(=\left(-1+3\right)+\left(-5+7\right)+...+\left(-97+99\right)\)
\(=2+2+...+2\)
\(=2\cdot25=50\)
e) \(1+\left(-2\right)+3+\left(-4\right)+...+1999+\left(-2000\right)+2001\)\(=\left[1+\left(-2\right)\right]+\left[3+\left(-4\right)\right]+...+\left[1999+\left(-2000\right)\right]+2001\)
\(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)+2001\)
\(=\left(-1\right)\cdot1000+2001=-1000+2001=1001\)
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Bài 4 :
a) \(\left(2ab^2\right):\left(abc\right)=\left[2\cdot4\cdot\left(-6^2\right)\right]:\left[4\cdot\left(-6\right)\cdot12\right]\)
\(=\left[2\cdot4\cdot36\right]:\left[4\cdot\left(-6\right)\cdot12\right]\)
\(=\left[8\cdot36\right]:\left[-24\cdot12\right]\)
\(=288:\left(-288\right)=-1\)
b) \(\left[\left(-25\right)\cdot\left(-27\right)\cdot\left(-x\right)\right]:y=\left[\left(-25\right)\cdot\left(-27\right)\cdot4\right]:\left(-9\right)\)
\(=\left[675\cdot4\right]:\left(-9\right)=2700:\left(-9\right)=-300\)
c) \(\left(a^2-b^2\right):\left(a+b\right)\left(a-b\right)=\left(5^2-\left(-3^2\right)\right):\left(5+\left(-3\right)\right)\left(5-\left(-3\right)\right)\)
\(=\left(25-9\right):\left(5+\left(-3\right)\right)\left(5-\left(-3\right)\right)\)
\(=16:2\cdot8=8\cdot8=64\)