400 km/p = \(\dfrac{400000}{60}\) m/s = \(\dfrac{20000}{3}\) m/s

q) \(6x^2-7x-20=\left(6x^2-15x\right)+\left(8x-20\right)=\left(2x-5\right)\left(3x+4\right)\)

r) \(x^2-4xy+3y^2=\left(x^2-xy\right)-\left(3xy-3y^2\right)=\left(x-y\right)\left(x-3y\right)\)

s) \(9x^2-9xy-4y^2=\left(9x^2+3xy\right)-\left(12xy+4y^2\right)=\left(3x+y\right)\left(3x-4y\right)\)

t) \(9x^4-10x^2+1=\left(9x^4-9x^2\right)-\left(x^2-1\right)=\left(9x^2-1\right)\left(x^2-1\right)=\left(3x-1\right)\left(3x+1\right)\left(x-1\right)\left(x+1\right)\)

v) \(4x^4+4x^2y^2-8y^4=\left(4x^4+4x^2y^2+y^2\right)-9y^2=\left(2x^2+y^2\right)^2-\left(3y\right)^2=\left(2x^2+y^2+3y\right)\left(2x^2+y^2-3y\right)\)

h) \(4x^2-4x-15=\left(4x^2-10x\right)+\left(6x-15\right)=\left(2x-5\right)\left(2x+3\right)\)

k) \(8x^2+10x-3=\left(8x^2-2x\right)+\left(12x-3\right)=\left(4x-1\right)\left(2x+3\right)\)

m) \(x^2-10x+24=\left(x^2-4x\right)-\left(6x-24\right)=\left(x-4\right)\left(x-6\right)\)

n) \(3x^2-7x+2=\left(3x^2-6x\right)-\left(x-2\right)=\left(x-2\right)\left(3x-1\right)\)

p) \(x^2-x-12=\left(x^2-4x\right)+\left(3x-12\right)=\left(x-4\right)\left(x+3\right)\)

Ta có \(L=\left\{1;3;5;7;9;11;...\right\}\)

4 số tự nhiên thuộc L là: 1; 3; 5; 7

2 số tự nhiên không thuộc L là: 4; 8

q) \(6x^2-7x-20=\left(6x^2-15x\right)+\left(8x-20\right)=\left(2x-5\right)\left(3x+4\right)\)

r) \(x^2-4xy+3y^2=\left(x^2-xy\right)-\left(3xy-3y^2\right)=\left(x-y\right)\left(x-3y\right)\)

s) \(9x^2-9xy-4y^2=\left(9x^2+3xy\right)-\left(12xy+4y^2\right)=\left(3x+y\right)\left(3x-4y\right)\)

t) \(9x^4-10x^2+1=\left(9x^4-9x^2\right)-\left(x^2-1\right)=\left(9x^2-1\right)\left(x^2-1\right)=\left(3x-1\right)\left(3x+1\right)\left(x-1\right)\left(x+1\right)\)

v) \(4x^4+4x^2y^2-8y^4=\left(4x^4+4x^2y^2+y^2\right)-9y^2=\left(2x^2+y^2\right)^2-\left(3y\right)^2=\left(2x^2+y^2+3y\right)\left(2x^2+y^2-3y\right)\)

h) \(4x^2-4x-15=\left(4x^2-10x\right)+\left(6x-15\right)=\left(2x-5\right)\left(2x+3\right)\)

k) \(8x^2+10x-3=\left(8x^2-2x\right)+\left(12x-3\right)=\left(4x-1\right)\left(2x+3\right)\)

m) \(x^2-10x+24=\left(x^2-4x\right)-\left(6x-24\right)=\left(x-4\right)\left(x-6\right)\)

n) \(3x^2-7x+2=\left(3x^2-6x\right)-\left(x-2\right)=\left(x-2\right)\left(3x-1\right)\)

p) \(x^2-x-12=\left(x^2-4x\right)+\left(3x-12\right)=\left(x-4\right)\left(x+3\right)\)