a: Đặt \(A=5x\left(x^2-4x+1\right)-\left(x^2+1\right)\left(5x-1\right)+20\left(1-x\right)\left(1+x\right)\)
\(=5x^3-20x^2+5x-\left(5x^3-x^2+5x-1\right)+20\left(1-x^2\right)\)
\(=5x^3-20x^2+5x-5x^3+x^2-5x+1+20-20x^2\)
\(=-39x^2+21\)
Khi x=-1 thì \(A=-39\cdot\left(-1\right)^2+21=-39+21=-18\)
b: Đặt \(B=\left(2-x\right)\left(x^2+2x+4\right)+x\left(x^2-1\right)+\frac12\left(8x-3\right)\)
\(=2^3-x^3+x^3-x+4x-\frac32\)
\(=3x+8-\frac32=3x+6,5\)
Khi x=1/6 thì \(B=3\cdot\frac16+6,5=0,5+6,5=7\)
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