Gọi số kg giấy của 3 lớp A; B ; C lần lượt là x; y; z ( >0 ; kg) 

Theo đề baì số kg giấy của 3 lớp A;B:C lần lượt tỉ lệ với: 3;5 ; 6 

=> \(\frac{x}{3}=\frac{y}{5}=\frac{z}{6}\)

Mặt khác số kg giấy của 7B nhiều hơn 7A là 18 kg 

=> y - x = 18 

Áp dụng dãy tỉ số bằng nhau ta có:

\(\frac{x}{3}=\frac{y}{5}=\frac{z}{6}=\frac{y-x}{5-3}=\frac{18}{2}=9\)

=> \(\hept{\begin{cases}\frac{x}{3}=9\\\frac{y}{5}=9\\\frac{z}{6}=9\end{cases}}\Leftrightarrow\hept{\begin{cases}x=27\\y=45\\z=54\end{cases}}\)( tm )

Kết luận: Số kg giấy thu gom được của 3 lớp A; B: C lần lượt là: 27kg ; 45kg ; 54 kg.

đáp án là 1614 nha

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Hok tốt nha bạn!!!

1.It's about 25km from my house to my grandparents' house

2.I used to ride a small bike in the yard outside the flat

3.The used to be a bus station in the city center but it was moved to the suburds

4.Children must learn about road side before they are allowed to ride a bike on the road

làm sai thì câm cha cái mồm lại đi

a) xét tam giác ABC zà tam giác  ACD có

AB=AD(gt)

AC chung

góc BAC= góc CAD =90 độ

=> 2 tam giác trên = nhau

=>\(\hept{\begin{cases}\widehat{DCA}=\widehat{ACB}\\DC=BC\end{cases}}\)

=>\(\hept{\begin{cases}\widehat{DCA}+\widehat{ACB}=30^0+30^0=60\\\Delta DBC\left(cân\right)\end{cases}}\)

=> \(\Delta DBC\)đều

b) ta có tam giác BAC cân

  tam giác BAD đều

=>\(\hept{\begin{cases}AC=AB\\BD=BC=DC\end{cases}}\)

=> \(\hept{\begin{cases}AC=AB\\\frac{1}{2}BD=\frac{1}{2}BC=\frac{1}{2}DC\end{cases}}\)

=> \(\hept{\begin{cases}AC=AB\\AB=AD=\frac{1}{2}BC=\frac{1}{2}DC\end{cases}}\)

=>\(AD=\frac{1}{2}BC=>BC=2AD\)

A B C H

Áp dụng định lý Pythagoras ta có:

\(AH^2+BH^2=AC^2\Rightarrow AC^2-BH^2=AH^2\)

\(AH^2+HC^2=AC^2\Leftrightarrow AC^2-HC^2=AH^2\)

Khi đó:

\(AC^2-BH^2=AC^2-HC^2\)

\(\Rightarrow AC^2+HC^2=AC^2+BH^2\)

=> ĐPCM

A B C H

Áp dụng định lí pitago cho \(\Delta\)AHC vuông tại H ta có:

\(AC^2=AH^2+CH^2\)

=> \(BH^2+AC^2=BH^2+AH^2+CH^2\)(1)

Áp dụng định lí pitago cho \(\Delta\)ABH vuông tại H ta có:
\(AB^2=AH^2+BH^2\)(2)

Từ (1); (2) => \(BH^2+AC^2=AB^2+CH^2\)( đpcm)

Bn ơi đây là môn ngữ văn