sai đề 100%

ko có quy luật

biết rồi mi

\(\dfrac{3.3}{20.23}+\dfrac{3.3}{23.26}+...+\dfrac{3.3}{77.80}\)

\(=3\left(\dfrac{3}{20.23}+\dfrac{3}{23.26}+...+\dfrac{3}{77.80}\right)\)

\(=3\left(\dfrac{1}{20}-\dfrac{1}{23}+\dfrac{1}{23}-\dfrac{1}{26}+...+\dfrac{1}{77}-\dfrac{1}{80}\right)\)

\(=3\left(\dfrac{1}{20}-\dfrac{1}{80}\right)\)

\(=3.\dfrac{3}{80}=\dfrac{9}{80}< 1\left(đpcm\right)\)

Vậy...

rất là hay

a/

31.64+7.30+146.31=31.(64+146)+7.30

=31.210+7.30=6510+210=6720

b/2³.3².28+2³.3⁵.8=2³.(3².28+3⁵.8)

=2³.(9.28+243.8)=8.(252+1944)

=8.2196=17568

=> 1+2+3+4+5+....+x = 190

x(x+1) = 190.2 = 380

x(x+1) = 19.(19 + 1)

VẬy x = 19 

3.3².3³.3⁴......3^x=3¹⁹⁰

=>3^{(1+2+3+...........+x)}=3¹⁹⁰

=>1+2+3+.........+x=190

=>\(\frac{x.\left(x+1\right)}{2}\)=180

=>x.(x+1)=190.2

=>x.(x+1)=380

=>x=19

\(3\cdot3^2\cdot3^3\cdot3^4\cdot....\cdot3^x=3^{190}\)

\(\Leftrightarrow3^{1+2+3+...+x}=3^{190}\)

\(\Leftrightarrow1+2+3+...+x=190\)

\(\Leftrightarrow\dfrac{x\left(x+1\right)}{2}=190\Leftrightarrow x\left(x+1\right)=380\)

\(\Leftrightarrow x^2+x-380=0\)

\(\Leftrightarrow x^2-19x+20x-380=0\)

\(\Leftrightarrow x\left(x-19\right)+20\left(x-19\right)=0\)

\(\Leftrightarrow\left(x-19\right)\left(x+20\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x-19=0\\x+20=0\end{matrix}\right.\)\(\Leftrightarrow x=19\left(x>0\right)\)

dễ

3.3^2.3^3.............3^x=3^190

3^1+2+3+4+....+x=3^190

nên 1+2+3+.........+x=190

hay (x+1).x :2 =190        nen 190.2= (x+1) . x hay 380 =19.20

vay x=19

Đáp số Ra 19 đó các bạn ạ

9

nhân nha mn

9