4/7

12/21=12:3/21:3=4/7

4/7

4/1

4

4

x.(x+4).(x-4)-(x²+1).(x²-1)

 =x.(x²-16)-(x⁴-1)

=x³-16x-x⁴+1

=-x⁴+x³-16x+1

 x.(x+4).(x-4)-(x²+1).(x²-1)

 =x.(x²-16)-(x⁴-1)

=x³-16x-x⁴+1

=x⁴+x³-16x+1

4/9

4/9

\(\dfrac{12}{27}\) sau khi rút gọn sẽ thành \(\dfrac{4}{9}\) vì 

\(\dfrac{12}{27}\) = \(\dfrac{12:3}{27:3}\) = \(\dfrac{4}{9}\)

Easy thật : 

\(a^3+b^3-\left(a^2-2ab+b^2\right)\left(a-b\right)\)

\(=a^3+b^3-\left(a-b\right)^2\left(a-b\right)\)

\(=a^3+b^3-\left(a-b\right)^3\)

Thay \(a=-4;b=4\)vào biểu thức , ta được : 

\(\left(-4\right)^3+4^3-\left(-4-4\right)^3\)

\(=8^3\)

\(=512\)

a³ + b³ - ( a² - 2ab + b² )( a - b )

= a³ + b³ - ( a - b )²( a - b )

= a³ + b³ - ( a - b )³

Thế a = -4 ; b = 4 ta được

(-4)³ + 4³ - ( -4 - 4 )³

= -64 + 64 - ( -512 )

= 512

\(\dfrac{1}{9}-\dfrac{8}{1}=\dfrac{1}{9}-\dfrac{72}{9}=\dfrac{-71}{9}\)

1/9- 8

= 1/9- 8/1

= 1/8- 72/9

= -71/9

\(\dfrac{5}{45}=\dfrac{5:5}{45:5}=\dfrac{1}{9}\);

\(\dfrac{40}{5}=\dfrac{40:5}{5:5}=\dfrac{8}{1}=8\)

\(\dfrac{1}{9}-8=\dfrac{1}{9}-\dfrac{72}{9}=-\dfrac{71}{9}\)

1.Ý A

\(P=cos^4x-sin^4x=\left(cos^2x-sin^2x\right)\left(cos^2x+sin^2x\right)=cos2x\)

2. Ý B

\(D=sin\left(\dfrac{5\pi}{2}-\alpha\right)+cos\left(13\pi+\alpha\right)-3sin\left(\alpha-5\pi\right)\)

\(=sin\left(2\pi+\dfrac{\pi}{2}-\alpha\right)+cos\left(\pi+\alpha+12\pi\right)-3sin\left(\alpha+\pi-6\pi\right)\)

\(=sin\left(\dfrac{\pi}{2}-\alpha\right)+cos\left(\pi+\alpha\right)-3sin\left(\alpha+\pi\right)\)

\(=cos\alpha-cos\alpha+3sin\alpha=3sin\alpha\)

( 4x + 4. 4 ) : 2 x 2

= 4x + 16

Hoq chak :3