Nhận thấy: \(5^{27}=\left(5^3\right)^9=125^9< 128^9=\left(2^7\right)^9=2^{63}\left(1\right)\)

và: \(2^{63}=\left(2^9\right)^7=512^7< 625^7=\left(5^4\right)^7=5^{28}\left(2\right)\)

\(\underrightarrow{\left(1\right);\left(2\right)}5^{27}< 2^{63}< 5^{28}\)

\(3^{39}< 3^{40}=\left(3^2\right)^{20}=9^{20}< 11^{20}< 11^{21}\)

\(27x^3-108x^2+144x-64\\ =\left(3x\right)^3-3.\left(3x\right)^2.4+3.3x.4^2-4^3\\ =\left(3x-4\right)^3\)

Cạnh hình vuông:

  416 x 2 = 832 (cm)

Chu vi HV mới tạo thành:

  832 x 4 = 3328 (cm)

  Đ/S:3328cm

Đáp án D bạn nhé

a) \(\dfrac{1}{2}\times\dfrac{2}{5}=\dfrac{1}{5}\)

b) \(25\%=\dfrac{25}{100}=\dfrac{1}{4}\)

c) 45,7

d) 120 + 15 = 135

\(60\%x+\dfrac{2}{3}x=-76\\ \Rightarrow\dfrac{60}{100}x+\dfrac{2}{3}x=-76\\ \Rightarrow\dfrac{3}{5}x+\dfrac{2}{3}x=-76\\ \Rightarrow\dfrac{9}{15}x+\dfrac{10}{15}x=-76\\ \Rightarrow\dfrac{19}{15}x=-76\\ x=-76:\dfrac{19}{15}\\ x=-60\)

Theo đề, suy ra: \(x-1\inƯC\left(12;28\right)\)

Mà: \(12=2^2.3,28=2^2.7\\ \RightarrowƯCLN\left(12;28\right)=2^2=4\)

\(\Rightarrow x-1\inƯ\left(4\right)=\left\{1;-1;2;-2;4;-4\right\}\Rightarrow x\in\left\{2;0;3;-1;5;-3\right\}\)

Sửa đề: \(\left(5x-3\right)\left(2x-c\right)=ax^2+bx+21\left(1\right)\) hoặc \(\left(5x-3\right)\left(2x-c\right)=ax^2+bx-21\left(2\right)\)

Kiểu đề 1:

\(\left(1\right)\Leftrightarrow10x^2-5xc-6x+3c=ax^2+bx+21\\ \Leftrightarrow10x^2+x\left(-5c-6\right)+3c=ax^2+bx+21\)

Đồng nhất hệ số: \(\left\{{}\begin{matrix}a=10\\-5c-6=b\\3c=21\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=10\\c=7\\-5.7-6=b\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=10\\c=7\\b=-41\end{matrix}\right.\)

Kiểu đề 2:

\(\left(2\right)\Leftrightarrow10x^2+x\left(-5c-6\right)+3c=ax^2+bx-21\)

Đồng nhất hệ số: \(\left\{{}\begin{matrix}a=10\\-5c-6=b\\3c=-21\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=10\\c=-7\\-5.\left(-7\right)-6=b\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=10\\c=-7\\b=29\end{matrix}\right.\)

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