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

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

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

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

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

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

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

Ta có: a + b = 1

M = a3 + b3 + 3ab(a2 + b2) + 6a2b2(a + b)

= (a + b)3 - 3ab(a + b) + 3ab[(a + b)2 - 2ab] + 6a2 b2 (a + b)

= 1 - 3ab + 3ab(1 - 2ab) + 6a2 b2

= 1 - 3ab + 3ab - 6a2 b2 + 6a2 b2

= 1
nhwos tick nha :D

M=a3+b3+3ab(a2+b2)+6a2b2(a+b)�=�3+�3+3��(�2+�2)+6�2�2(�+�)

Biến đổi:

a2+b2=a2+2ab+b2−2ab=(a+b)2−2ab�2+�2=�2+2��+�2−2��=(�+�)2−2��

a3+b3=(a+b)(a2−ab+b2)�3+�3=(�+�)(�2−��+�2)

Thay a+b=1�+�=1 và phần biến đổi vào biểu thức, ta được:

M=(a+b)(a2−ab+b2)+3ab.[(a+b)2−2ab]+6a2b2�=(�+�)(�2−��+�2)+3��.[(�+�)2−2��]+6�2�2

⇒M=a2−ab+b2+3ab.[1−2ab]+6a2b2⇒�=�2−��+�2+3��.[1−2��]+6�2�2

⇒M=a2−ab+b2+3ab−6a2b2+6a2b2⇒�=�2−��+�2+3��−6�2�2+6�2�2

⇒M=a2+2ab+b2⇒�=�2+2��+�2

⇒M=(a+b)2⇒�=(�+�)2

⇒M=1