-△ABC đều, đường cao AH.

\(\Rightarrow BH=CH=\dfrac{1}{2}BC=\dfrac{1}{2}a\)

-△ABH vuông tại H \(\Rightarrow AH^2+BH^2=AB^2\Rightarrow AH=\sqrt{a^2-\dfrac{1}{4}a^2}=\dfrac{\sqrt{3}}{2}a\)

-\(S_{ABC}=\dfrac{1}{2}.AH.BC=\dfrac{1}{2}.\dfrac{\sqrt{3}}{2}a.a=\dfrac{\sqrt{3}}{4}a^2\)

đáp án ở bên trên hả bạn

50/ D
4/ I like durians so much and so does my father.

Bài nào ạ

 31 - [ 26 - ( 209 + 35 ) ] 

= 31 - ( 26 - 344 )

=31 - ( -318)

= 31 + 318 ( trừ trừ thành cộng nha )

= 349

31-(26-(209+35)=31-

hok tốt

k cho mik

kb nữa nhé

Sao bạn trả lời giữa chừng thía 

750 x 3 - (1/3 x 960 + 960)=970

Số thứ hai là:960 : 3 = 320

Tổng  của 3số là:750 x 3 = 2250

Số thứ ba là:2250 - (960 + 320) = 970

Mình không nghĩ ra nên bạn tham khảo nhé :

Nguồn : http://gdthhatinh.violet.vn/

Theo bài ra \(\frac{2}{5}\)số gà ít hơn \(\frac{2}{3}\)số vịt là 200 con

Vậy,  \(\frac{6}{5}\)số gà ít hơn 2 lần số vịt là:

200 x 3 = 600 (con)

Hay \(\frac{3}{5}\)số gà ít hơn vịt là

600 : 2 = 300 con (1)

Mặt khác, \(\frac{3}{4}\)số gà nhiều hơn  \(\frac{5}{12}\)số vịt là 125 con

Nên 9 lần số gà nhiều hơn 5 lần số vịt là:

125 x 12 = 1500 (con)

Hay \(\frac{9}{5}\)số gà nhiều hơn số vịt là:

1500 : 5 = 300 con (2)

Từ (1) và (2) ta có:

\(\frac{9}{5}\)số gà nhiều hơn \(\frac{3}{5}\)số gà số con là

300 + 300 = 600 (con)

Phân số chỉ số phần của 600 con gà là:

 \(\frac{9}{5}-\frac{3}{5}=\frac{6}{5}\)

Số gà là:

600 x 5 : 6 = 500 (con)

Số vịt là:

3 x 500:5 + 300 = 600 (con)

Đáp số: 500 con gà

            600 con vịt

500 con gà 

600con vịt 

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