a: \(B\left(x\right)=5x\left(4x^2-2x+1\right)-2x\left(10x^2-5x-2\right)\)
\(=20x^3-10x^2+5x-20x^3+10x^2+4x\)
=9x
Khi x=15 thì \(B\left(x\right)=9\cdot15=135\)
b: \(A\left(x\right)=2x\left(3x^2-5x+8\right)-3x^2\left(2x-5\right)-16x\)
\(=6x^3-10x^2+16x-6x^3+15x^2-16x\)
\(=5x^2\)
Khi x=-5 thì \(A\left(x\right)=5\cdot\left(-5\right)^2=5\cdot25=125\)
c: \(C\left(x\right)=2x\left(3x^2-5x+8\right)-3x^2\left(2x-5\right)-16x\)
\(=6x^3-10x^2+16x-6x^3+15x^2-16x\)
\(=5x^2\)
Khi x=-1/9 thì \(C\left(x\right)=5\cdot\left(-\dfrac{1}{9}\right)^2=5\cdot\dfrac{1}{81}=\dfrac{5}{81}\)
*Sửa đề:
a)
\(B(x)=5x.(4x^2-2x+1)-2x.(10x^2-5x-2)\\=20x^3-10x^2+5x-20x^3+10x^2+4x\\=(20x^3-20x^3)+(-10x^2+10x^2)+(5x+4x)\\=9x\)
\(\Rightarrow B\left(15\right)=9.15=135\)
b)
\(A(x)=2x.(3x^2-5x+8)-3x^2.(2x-5)-16x\\=6x^3-10x^2+16x-6x^3+15x^2-16x\\=(6x^3-6x^3)+(-10x^2+15x^2)+(16x-16x)\\=5x^2\\\Rightarrow A(-5)=5.(-5)^2=5^3=125\)
c)
\(C\left(x\right)=A\left(x\right)=5x^2\\ \Rightarrow C\left(\dfrac{-1}{9}\right)=5\cdot\left(\dfrac{-1}{9}\right)^2=\dfrac{5}{81}\)
#$\mathtt{Toru}$
