Tìm x, biết
4x\5+4x\45+4x\117+...+4x\1221=72\37
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
a, 72-(4.x+15)=45 con cau b khong lam dc
4.x+15=72-45
4.x+15=27
4.x=27-15
4.x=8
x=8:4
x=2
\(a,\Leftrightarrow x-1=4\Leftrightarrow x=5\\ b,\Leftrightarrow\left\{{}\begin{matrix}x\ge\dfrac{3}{4}\\3x+1=4x-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x\ge\dfrac{3}{4}\\x=4\left(tm\right)\end{matrix}\right.\Leftrightarrow x=4\\ c,ĐK:x\ge-5\\ PT\Leftrightarrow2\sqrt{x+5}-3\sqrt{x+5}+4\sqrt{x+5}=6\\ \Leftrightarrow3\sqrt{x+5}=6\\ \Leftrightarrow\sqrt{x+5}=3\\ \Leftrightarrow x+5=9\\ \Leftrightarrow x=4\left(tm\right)\)
\(d,\Leftrightarrow\sqrt{\left(x-2\right)^2}=\sqrt{\left(\sqrt{5}+1\right)^2}\\ \Leftrightarrow\left|x-2\right|=\sqrt{5}+1\\ \Leftrightarrow\left[{}\begin{matrix}x-2=\sqrt{5}+1\\2-x=\sqrt{5}+1\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\sqrt{5}+3\\x=1-\sqrt{5}\end{matrix}\right.\)
Lời giải:
$2x^2+4x+2=72$
$\Leftrightarrow 2(x^2+2x+1)=72$
$\Leftrightarrow 2(x+1)^2=72$
$\Leftrightarrow (x+1)^2=36$
$\Rightarrow x+1=\pm 6$
$\Rightarrow x=5$ hoặc $x=-7$
a ) \(4x+5y=76\Leftrightarrow5y=76-4x=4\left(19-x\right)\)
Do \(4\left(19-x\right)⋮4\forall x\in N\) nên \(5y⋮4\) mà \(\left(4;5\right)=1\) nên \(y⋮4\) (1)
Từ \(4x+5y=76\forall x;y\in N\Rightarrow5y\le76\Rightarrow y\le\frac{76}{5}=15,2\) (2)
Từ (1) ; (2) \(\Rightarrow y=\left\{0;4;8;12\right\}\)
Với \(y=0\Rightarrow4x=76\Rightarrow x=19\left(TM\right)\)
Với \(y=4\Rightarrow4x+20=76\Rightarrow x=\frac{76-20}{4}=14\left(TM\right)\)
Với \(y=8\Rightarrow4x+40=76\Rightarrow x=\frac{76-40}{4}=9\left(TM\right)\)
Với \(y=12\Rightarrow4x+60=76\Rightarrow x=\frac{76-60}{4}=7\left(TM\right)\)
Vậy \(\left(x;y\right)=\left\{\left(0;19\right);\left(14;4\right);\left(9;8\right);\left(7;12\right)\right\}\)
(4x-3).(4x+2) + (4x+5).(1-4x) = 2.52
16x2 + 8x - 12x - 6 + 4x - 16x2 + 5 - 20x = 50
(16x2 - 16x2) + ( 8x-12x+4x-20x) - (6-5) = 50
-20x = 50
x = -5/2
Sai rồi kìa! Kém!
\(\dfrac{1}{1.5}=\dfrac{1}{5}\)
\(1-\dfrac{1}{5}=\dfrac{4}{5}\)
\(\Rightarrow\dfrac{4}{5}\ne\dfrac{1}{5}\Rightarrow\dfrac{1}{1.5}\ne1-\dfrac{1}{5}\)
Tốt nhất là nên xóa đi làm lại kẻo người ta cười cho!
Gợi ý: Đến cái đoạn sai...
\(\Leftrightarrow4x\left(\dfrac{1}{1.5}+\dfrac{1}{5.9}+\dfrac{1}{9.13}+...+\dfrac{1}{33.37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x.\dfrac{1}{4}\left(\dfrac{4}{1.5}+\dfrac{4}{5.9}+\dfrac{4}{9.13}+...+\dfrac{4}{33.37}\right)=...\)
\(\Leftrightarrow4x.\dfrac{1}{4}\left(1-\dfrac{1}{5}+\dfrac{1}{5}-....\right)\) (tự làm tiếp)
\(\dfrac{4x}{5}+\dfrac{4x}{45}+\dfrac{4x}{117}+...+\dfrac{4x}{1221}=\dfrac{72}{37}\)
\(\dfrac{4x}{1.5}+\dfrac{4x}{5.9}+\dfrac{4x}{9.13}+...+\dfrac{4x}{33.37}=\dfrac{72}{37}\)
\(\Leftrightarrow4x\left(\dfrac{1}{1.5}+\dfrac{1}{5.9}+\dfrac{1}{9.13}+...+\dfrac{1}{33.37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x\left(1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+...+\dfrac{1}{33}-\dfrac{1}{37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x\left(1-\dfrac{1}{37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x.\dfrac{36}{37}=\dfrac{72}{37}\)
\(4x=\dfrac{72}{37}:\dfrac{36}{37}\)
\(4x=2\)
\(x=2:4\)
\(x=\dfrac{1}{2}\)