Câu 4:
a: Ta có: \(\left(2x-5\right)\left(2x+5\right)-\left(2x-3\right)^2-12x\)
\(=4x^2-25-4x^2+12x-9-12x\)
=-34
b: Ta có: \(\left(2y-1\right)^3-2y\left(2y-3\right)^2-6y\left(2y-2\right)\)
\(=8y^3-12y^2+6y-1-2y\left(4y^2-12y+9\right)-12y^2+12y\)
\(=8y^3-24y^2+18y-1-8y^3+24y^2-18y\)
=-1
Câu 3:
1: Ta có: \(A=x\left(x-2\right)\left(x+2\right)-\left(x-3\right)\left(x^2+3x+9\right)\)
\(=x^3-4x-x^3+27\)
\(=-4x+27\)
\(=-4\cdot\dfrac{1}{4}+27\)
\(=-1+27=26\)
2: Ta có: \(\left(4x+1\right)\left(16x^2-4x+1\right)-16x\left(4x^2-5\right)=17\)
\(\Leftrightarrow64x^3+1-64x^3+80x=17\)
\(\Leftrightarrow80x=16\)
hay \(x=\dfrac{1}{5}\)
3: Ta có: \(Q=\left(a^2-1\right)\left(a^2-a+1\right)\left(a^2+a+1\right)\)
\(=\left(a^3-1\right)\left(a^3+1\right)\)
\(=a^6-1\)
