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\(A=\dfrac{30}{4x7}+\dfrac{30}{7x10}+\dfrac{30}{10x13}+...+\dfrac{30}{37x40}\)
\(=10x\left(\dfrac{1}{4}-\dfrac{1}{7}\right)+10x\left(\dfrac{1}{7}-\dfrac{1}{10}\right)+...+10x\left(\dfrac{1}{37}-\dfrac{1}{40}\right)\)
=10x\(\left(\dfrac{1}{4}-\dfrac{1}{40}\right)=\dfrac{10x9}{10x4}=\dfrac{9}{4}\)
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Đáp án đúng là:
(600 + 30) + 5 = 600 + (30 + 5)
(600 x30) x 5 = 600 x ( 30 x 5)
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\(\frac{1}{30}+\frac{2}{30}+...+\frac{9}{30}\)
\(=\frac{1+2+...+9}{30}\)
\(=\frac{\left(9+1\right)\cdot9\div2}{30}\)
\(=\frac{45}{30}=\frac{3}{2}\)
\(\frac{1}{30}+\frac{2}{30}+...+\frac{8}{30}+\frac{9}{30}\)
\(\Rightarrow\frac{9\left(\frac{9+1}{2}\right)}{30}\)
\(\Rightarrow\frac{9\cdot5}{30}=\frac{45}{30}\)
\(\Rightarrow\frac{3}{2}\)
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30 % của 90 là : 90 x 30 : 100 = 27
=27 tích cho tớ lên 140 cái