Chọn A

\(37,5.8,5-7,5.3,4-6,6.7,5+1,5.37,5\)

\(=(37,5.8,5+1,5.37,5)-(7,5.3,4+6,6.7,5)\)

\(=37,5.\left(8,5+1,5\right)-7,5\left(3,4+6,6\right)\)

\(=37,5.10-7,5.10=10.\left(37,5-7,5\right)=10.30=300\)

= (37,5.8,5 + 15 . 37,5 )-(7,5 . 3.4 + 6,6 .7,5)

= 37,5(8,5 + 15) - 7,5 (3,4 +6,6 )

=881,25 - 75 = 806,25

\(\left(x+5\right)\left(x-5\right)-x\left(x-10\right)=5\)

\(=>x^2-25-x^2+10x=5\)

\(=>10x=30\)

\(=>x=3\)

Đề gì mới đúng nha : \(\left(x-5\right)\left(x+5\right)-x\left(x-10\right)=5\)

Làm :

\(\left(x-5\right)\left(x+5\right)-x\left(x-10\right)=5\)

\(\Leftrightarrow x^2-5^2-x^2+10x=5\)

\(\Leftrightarrow10x-25=5\)

\(\Leftrightarrow10x=30\)

\(\Leftrightarrow x=30:10\)

\(\Leftrightarrow x=3\)

\(x^2+10x+25=x^2-25\)

\(10x=-50\)

\(x=-5\)

(x+5)^2 = (x-5)(x+5)

<=>x^2+10x+25 = x^2-25

<=>x^2-x^2+25+25=-10x

<=>x=(-5)

a/ \(A=\left(x-5\right)^2+\left(2x+1\right)^2-2\left(2x^2+8,5\right)\)

\(=\left(x^2-2.x.5+5^2\right)+\left[\left(2x\right)^2+2.2x.1+1^2\right]-2.2x^2-2.8,5\)

\(=\left(x^2-10x+25\right)+\left(4x^2+4x+1\right)-4x^2-17\)

\(=x^2-10x+25+4x^2+4x+1-4x^2-17\)

\(=x^2-6x+9\)

\(=x^2-2.3x+3^2\)

\(=\left(x-3\right)^2\)

b/ A = 0

\(\Rightarrow\left(x-3\right)^2=0\)

\(\Rightarrow x-3=0\)

\(\Rightarrow x=0+3=3\)

c/ A = 36

\(\Rightarrow\left(x-3\right)^2=36\)

\(\Rightarrow\left(x-2\right)^2=\left(\pm6\right)^2\)

\(\Rightarrow\left[{}\begin{matrix}x-2=6\\x-2=-6\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=6+2=8\\x=-6+2=-4\end{matrix}\right.\)

Vậy................