Bài 6:
a) Ta có: \(P=\left(x+2\right)^3+\left(x-2\right)^3-2x\left(x^2+12\right)\)
\(=x^3+6x^2+12x+8+x^3-6x^2+12x-8-2x^3-24x\)
\(=0\)
b) Ta có: \(Q=\left(x-1\right)^3-\left(x+1\right)^3+6\left(x+1\right)\left(x-1\right)\)
\(=x^3-3x^2+3x-1-x^3-3x^2-3x-1+6\left(x^2-1\right)\)
\(=-6x^2-2+6x^2-6\)
=-8
Bài 5:
a) Ta có: \(\left(a+b\right)^3+\left(a-b\right)^3-6a^2b\)
\(=a^3+3a^2b+3ab^2+b^3+a^3-3a^2b+3ab^2-b^3-6a^2b\)
\(=2a^3\)
b) Ta có: \(\left(a+b\right)^3-\left(a-b\right)^3-6a^2b\)
\(=a^3+3a^2b+3ab^2+b^3-a^3+3a^2b-3ab^2+b^3-6a^2b\)
\(=2b^3\)
Bài 2:
1) \(11^3=\left(10+1\right)^3=1000+300+30+1=1331\)
2) \(9^3=\left(10-1\right)^3=1000-300+30-1=729\)
Bài 4:
a) Ta có: \(A=x^3+15x^2+75x+125\)
\(=\left(x+5\right)^3\)
\(=\left(-10+5\right)^3=\left(-5\right)^3=-125\)
q) Ta có: \(Q=x^3+\dfrac{3}{10}x^2+\dfrac{3}{100}x\)
\(=\left(\dfrac{9}{10}\right)^3+\dfrac{3}{10}\cdot\dfrac{81}{100}+\dfrac{3}{100}\cdot\dfrac{9}{10}\)
\(=\dfrac{729}{1000}+\dfrac{243}{1000}+\dfrac{27}{1000}\)
\(=\dfrac{999}{1000}\)
giúp mình bài 4 câu B,C D với ạ
