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Lời giải :
1. \(\left(\frac{1}{2}a+b\right)^3+\left(\frac{1}{2}a-b\right)^3\)
\(=\frac{a^3}{8}+\frac{3a^2b}{4}+\frac{3ab^2}{2}+b^3+\frac{a^3}{8}-\frac{3a^2b}{4}+\frac{3ab^2}{2}-b^3\)
\(=\frac{a^3}{4}+3ab^2\)
Lời giải :
2. \(x^3-3x^2+3x-1=0\)
\(\Leftrightarrow\left(x-1\right)^3=0\)
\(\Leftrightarrow x-1=0\)
\(\Leftrightarrow x=1\)
Vậy...
1) \(\left(\frac{1}{2}a+b\right)^3+\left(\frac{1}{2}a-b\right)^3\)
\(=\left(\frac{a}{2}+b\right)^2+\left(\frac{a}{2}-b\right)^2\)
\(=\left(\frac{a}{2}+b\right)\left[\left(\frac{a}{2}\right)^2+2.\frac{a}{b}b+b^2\right]+\left(\frac{a}{2}-b\right)\left[\left(\frac{a}{2}\right)^2-2.\frac{a}{2}b+b^2\right]\)
\(=\frac{a}{2}\left[\left(\frac{a}{2}\right)^2+2.\frac{a}{2}b+b^2\right]+b\left[\left(\frac{a}{2}\right)^2+2.\frac{a}{2}b+b^2\right]+\frac{a}{2}\left[\left(\frac{a}{2}\right)^2-2.\frac{a}{2}b+b^2\right]\)\(-b\left[\left(\frac{a}{2}\right)^2-2.\frac{a}{2}b+b^2\right]\)
\(=\frac{a^3}{8}+\frac{a^2b}{2}+\frac{ab^2}{2}+\frac{ba^2}{4}+b^2a+b^3+\frac{a^3}{8}-\frac{a^2b}{2}+\frac{ab^2}{2}-\frac{ba^2}{4}+b^2a-b^3\)
\(=\frac{a^3}{4}+3ab^2\)
2) \(x^3-3x^2+3x-1=0\)
\(\Leftrightarrow x^3-3x^2.1+3.x.1^2-1^3=0\)
\(\Leftrightarrow\left(x+1\right)^3=0\)
\(\Leftrightarrow x+1=0\)
\(\Leftrightarrow x=0-1\)
\(\Rightarrow x=-1\)
3) \(A=\left(4x-1\right)^3-\left(4x-3\right)\left(16x^2+3\right)\)
\(A=64x^3-32x^2+4x-16x^2+8x-1-64x^3-12x+48x^2+9\)
\(A=8\)
Vậy: biểu thức không phụ thuộc vào biến
1) \(\left(x+5\right)^3-x^3-125\)
\(=\left(x+5\right)\left(x^2+2x.5+5^2\right)-x^3-125\)
\(=x\left(x^2+2x.5+5^2\right)+5\left(x^2+2x.5+5^2\right)-x^3-125\)
\(=x^3+10x^2+25x+5x^2+50x+125-x^3-125\)
\(=15x^2+75x\)
2) \(\left(x-2\right)^3+6\left(x+1\right)^2-x^3+12=0\)
\(\Leftrightarrow x^3-4x^2+4x-2x^2+8x-8+6x^2+12x+6-x^3+12=0\)
\(\Leftrightarrow24x+10=0\)
\(\Leftrightarrow24x=0-10\)
\(\Leftrightarrow24x=-10\)
\(\Leftrightarrow x=-\frac{10}{24}=-\frac{5}{12}\)
\(\Rightarrow x=-\frac{5}{12}\)
3) \(\left(x-1\right)^3-x^3+3x^2-3x+1\)
\(=\left(x-1\right)\left(x^2-2x+1\right)-x^3+3x^2-3x+1\)
\(=x\left(x^2-2x+1\right)-\left(x^2-2x+1\right)-x^3+3x^2-3x+1\)
\(=x^3-2x^2+x-x^2+2x-1-x^3-3x^2-3x+1\)
\(=0\)
Vậy: biểu thức không phụ thuộc vào biến
A = (x - 1)(x + 3) - (x - 2)(5x - 4)
A = x2 + 2x - 3 - 5x2 + 14x - 8
A = -4x2 + 16x - 11
B = (3a - 2b)(9a2 + 6ab - 4b2)
B = 27a3 + 18a2b - 12ab2 - 18a2b - 12ab2 + 8b3
B = 27a3 -24ab2 + 8b3
C = (x - 1)(x + 1) - (2x - 3)(4 - 5x)
C = x2 - 1 - 8x + 10x + 12 - 15x
C = x2 - 13x + 11