`a,`

`(x+y+z)^2=[(x+y)+z]^2 =(x+y)^2 +2 (x+y)z +z^2 = x^2 +y^2 +z^2 +2xy+ 2yz +2zx` (Bằng VP)

`->` Đpcm

`b,`

`(x-y-z)^2=[(x-y)-z]^2=(x-y)^2 - 2 (x-y)z +z^2 = x^2 + y^2 +z^2 -2xy - 2zx -2yz` (Bằng VP)

`->` Đpcm

1) \(2x\left(x-5\right)+\left(x-2\right)\left(x+3\right)=2x^2-10x+x^2+3x-2x-6=3x^2-9x-6\)

2) \(\left(2x-5\right)\left(1-x\right)-\left(x-3\right)\left(-2x\right)=2x-2x^2-5+5x+2x^2-6x=x-5\)

3) \(\left(4x-3\right)\left(4x-3\right)-\left(3x+2\right)\left(3x-2\right)=\left(4x-3\right)^2-9x^2+4=16x^2-24x+9-9x^2+4\)

\(=7x^2-24x+13\)

4) \(\left(2x-1\right)\left(2x+1\right)\left(2x+1\right)-4\left(x^2+1\right)=\left(2x-1\right)[\left(2x+1\right)^2]-4x^2-4\)

\(=\left(2x-1\right)\left(4x^2+4x+4\right)-4x^2-4=8x^3+8x^2+8x-4x^2-4x-4-4x^2-4=8x^3+4x-8\)

5) \(3x\left(2x-8\right)-\left(2-6x\right)\left(5+x\right)=6x^2-24x-10-2x+30x+6x^2=12x^2+4x-10\)

6) \(x\left(3x-18\right)-3\left(x-4\right)\left(x-2\right)+8=3x^2-18x-3x^2+6x+12x-24+8=-16\)

7) \(\left(x+2\right)\left(x^2-2x+4\right)-x^2\left(x-2\right)-2x^2=x^3+8-x^3+2x^2-2x^2=8\)

\(M=4x-x^2+3\\ =-(x^2-4x-3)\\ =-(x^2-4x+4)+7\\ =-(x+2)^2+7 \leq7,\forall x\in \mathbb{R}\quad (\mathrm{vì}-(x+2)^2\leq0)\)

Dấu bằng xảy ra khi và chỉ khi \(-(x+2)^2=0\Leftrightarrow x+2=0 \Leftrightarrow x=-2\). 

Vậy \(\mathrm{Max}M=7\Leftrightarrow x=-2\).

Giải cả cách hộ mk

Theo bài ra ta có : a = 5k + 4

Khi đó : a2 = ( 5k + 4 )2

=> a2 = 25 k2 + 40k + 16

=> a2 = 5 . ( 5k2 + 8k + 3 ) + 1

Suy ra a2 chia cho 5 dư 1 ( ĐPCM )

Đpcm nha

Bài 1

a,=10201               b,39601              c,2491

Bài 2

(2x+3y)^2  +2(3x+3y)+1=(2x+3y+1)^2

Bài `1.`

`a, 101^2=(100+1)^2=100^2 +2.100.1 +1^2=10201`

`b, 199^2=(200-1)^2=200^2 - 2 . 200.1 +1^2=39601`

`c, 47 . 53=(50 - 3)(50+3) = 50^2 - 3^2=2491`

Bài `2.`

`(2x+3y)^2 +2 (2x+3y) +1 = (2x+3y)^2 +2 . (2x+3y).1+1^2=(2x+3y+1)^2`