\(4(x-1)^2-25(2-3x)\\=4(x^2-2x+1)-50+75x\\=4x^2-8x+4-50+75x\\=4x^2+67x-46\)

cảm ơn bn nhìu

a) \(\left(2x+5\right)^2-8^2\)

\(=\left(2x+5-8\right)\left(2x+5+8\right)\)

\(=\left(2x-3\right)\left(2x+13\right)\)

b) \(\left(2x-1\right)^2-\left(3x-1\right)^2\)

\(=\left[\left(2x-1\right)-\left(3x-1\right)\right]\left[\left(2x-1\right)+\left(3x-1\right)\right]\)

\(=\left(2x-1-3x+1\right)\left(2x-1+3x-1\right)\)

\(=-x\left(5x-2\right)\)

\(A=\dfrac{bc}{8a^2}+\dfrac{ca}{b^2}+\dfrac{ab}{c^2}\)

\(=\dfrac{\left(bc\right)^3+8\left(ca\right)^3+8\left(ab\right)^3}{8\left(abc\right)^2}\)

\(=\dfrac{\left(bc\right)^3+\left(2ca\right)^3+\left(2ab\right)^3}{8\left(abc\right)^2}\)

\(=\dfrac{\left(bc\right)^3+\left(2ab+2ca\right)^3-3.2ca.2ab\left(2ab+2ca\right)}{8\left(abc\right)^2}\)

\(=\dfrac{\left(bc\right)^3+\left(-bc\right)^3-3.2ca.2ab.\left(-bc\right)}{8\left(abc\right)^2}\)

\(=\dfrac{12\left(abc\right)^2}{8\left(abc\right)^2}=\dfrac{12}{8}\)

kkkk

Ta có: 126²=126.126=(123+3).126=123.126+3.126=123.126+378

           123.129= 123.(126+3)=123.126+123.3=123.126+369

       Vì 378>369 nên 123.126+378>123.126+369

⇒ 126²>123.129 hay 123.129<126²