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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 ) \(4x+5y=76\Leftrightarrow5y=76-4x=4\left(19-x\right)\)
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 \(\left(x;y\right)=\left\{\left(0;19\right);\left(14;4\right);\left(9;8\right);\left(7;12\right)\right\}\)
a) \(70-5\left(2x-3\right)=45\)
\(70-10x+15=45\)
\(-10x=45-70-15\)
\(-10x=-40\)
\(x=4\)
vay \(x=4\)
b) \(156-\left(x+61\right)=82\)
\(x+61=156-82\)
\(x+61=74\)
\(x=74-61\)
\(x=13\)
vay \(x=13\)
c) \(6\left(5x+35\right)=330\)
\(30x+210=330\)
\(30x=330-210\)
\(30x=120\)
\(x=4\)
vay \(x=4\)
d) \(936-\left(4x+24\right)=72\)
\(4x+24=936-72\)
\(4x+24=864\)
\(4x=864-24\)
\(4x=840\)
\(x=210\)
vay \(x=210\)
\(70-5.\left(2x-3\right)\)\(=45\)
\(5.\left(2x-3\right)\)\(=\)\(70-45\)
\(5.\left(2x-3\right)\)\(=25\)
\(2x-3=25:5\)
\(2x-3=5\)
\(2x=8\)
\(x=8:2\)
\(x=4\)
\(156-\left(x+61\right)\)\(=82\)
\(x+61=156-82\)
\(x+61=74\)
\(x=74-61\)
\(x=13\)
\(6.\left(5x+35\right)\)\(=330\)
\(5x+35=330:6\)
\(5x+35=55\)
\(5x=55-35\)
\(5x=20\)
\(x=20:5\)
\(x=4\)
\(936-\left(4x+24\right)\)\(=72\)
\(4x+24=936-72\)
\(4x+24=864\)
\(4x=864-24\)
\(4x=840\)
\(x=840:4\)
\(x=210\)
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)
\(\dfrac{4x}{5}+\dfrac{4x}{45}+\dfrac{4x}{117}+...+\dfrac{4x}{1221}=\dfrac{72}{37}\)
\(\dfrac{4x}{1.5}+\dfrac{4x}{5.9}+\dfrac{4x}{9.13}+...+\dfrac{4x}{33.37}=\dfrac{72}{37}\)
\(\Leftrightarrow4x\left(\dfrac{1}{1.5}+\dfrac{1}{5.9}+\dfrac{1}{9.13}+...+\dfrac{1}{33.37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x\left(1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+...+\dfrac{1}{33}-\dfrac{1}{37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x\left(1-\dfrac{1}{37}\right)=\dfrac{72}{37}\)
\(\Leftrightarrow4x.\dfrac{36}{37}=\dfrac{72}{37}\)
\(4x=\dfrac{72}{37}:\dfrac{36}{37}\)
\(4x=2\)
\(x=2:4\)
\(x=\dfrac{1}{2}\)