Chiều cao hình thang là:
`(20+10):2=15`(m)

Diện tích hình thang là:

`(20+10)xx15:2=225`(m²)

Số thóc thu được trên thửa ruộng là:

`225:100xx50=112,5`(kg)

số tiền bán thóc là:
`112,5:1xx5500=618750`(đồng)

a, `12 + 126 + 42 + 74 + 158 + 88`

`= (12+88)+(126+74)+(42+158)`

`=100+200+200`

`=500`

b, `15xx23+15xx56+15xx21`

`=15xx(23+56+21)`

`=15xx100`

`=1500`

Đề sai r bạn

C

\(\dfrac{4}{5}\times\dfrac{2}{7}+\dfrac{4}{5}\times\dfrac{5}{7}\\ =\dfrac{4}{5}\times\left(\dfrac{2}{7}+\dfrac{5}{7}\right)\\ =\dfrac{4}{5}\times\dfrac{7}{7}\\ =\dfrac{4}{5}\times1\\ =\dfrac{4}{5}\)

d, Dễ dàng cm đc \(\Delta BHA\sim\Delta BAC\left(g.g\right)\)

\(\Rightarrow\dfrac{BH}{BA}=\dfrac{AB}{BC}\\ \Rightarrow AB^2=BH.BC=BH.2.BK\\ \Rightarrow\dfrac{1}{BH}=\dfrac{2BK}{AB^2}\\ \Rightarrow\dfrac{BK}{BH}=2\left(\dfrac{BK}{AB}\right)^2\\ \Rightarrow\dfrac{BK}{BH}-1=2\left(\dfrac{BK}{AB}\right)^2-1\\ \Rightarrow\dfrac{HK}{BH}=2\left(\dfrac{BK}{AB}\right)^2-1\left(đpcm\right)\)

Xin 5 coin

1.D

2.B

3.D

4.C

5.A

\(1,\\ a,\dfrac{3}{4}+\dfrac{5}{2}=\dfrac{3}{4}+\dfrac{10}{4}=\dfrac{13}{4}\\ b,3+\dfrac{6}{11}=\dfrac{33}{11}+\dfrac{6}{11}=\dfrac{39}{11}\\ c,2-\dfrac{5}{7}=\dfrac{14}{7}-\dfrac{5}{7}=\dfrac{9}{7}\\ d,\dfrac{11}{3}-2=\dfrac{11}{3}-\dfrac{6}{3}=\dfrac{5}{3}\\ 2,\\ a,\dfrac{13}{19}+\dfrac{18}{19}+\dfrac{17}{19}=\dfrac{13+18+17}{19}=\dfrac{48}{49}\\ b,\dfrac{3}{5}+\dfrac{3}{16}+13+16=\dfrac{48}{80}+\dfrac{15}{80}+\dfrac{1040}{80}+\dfrac{1280}{80}=\dfrac{2383}{80}\)

3, Chiều rộng hcn là:
`4:2- 5/6= 7/6`(m)

Đáp số: `7/6`m

\(S_{ABC}=\dfrac{AB.AC}{2}=\dfrac{21.28}{2}=294\left(cm^2\right)\)

Ta có:\(S_{ABC}=\dfrac{AB.AC}{2}\) mà ta lại có: \(S_{ABC}=\dfrac{AH.BC}{2}\)

\(\Rightarrow\dfrac{AB.AC}{2}=\dfrac{AH.BC}{2}\Rightarrow AB.AC=AH.BC\left(đpcm\right)\)