There are the following activities to buy books: for those who buy 1-19 books, 10% discount is allowed for each book; for those who buy more than 20 books (including 20 Books), 70% discount is allowed for each book. It is known that the price of each book is 5 yuan. For those who buy 30 books twice, the total cost is 129 yuan

There are the following activities to buy books: for those who buy 1-19 books, 10% discount is allowed for each book; for those who buy more than 20 books (including 20 Books), 70% discount is allowed for each book. It is known that the price of each book is 5 yuan. For those who buy 30 books twice, the total cost is 129 yuan

If every time you buy a book is less than 20, there will be a result of 30 * 4.5 = 135, then there must be a time greater than 20, a time less than 20, set an X book, a time y book, x + y = 30; (1) 3.5x + 4.5y = 129; (2) (1) equation equal sign around * 3.5, get 3

Please answer me quickly ~ scientific notation~
Using scientific notation to express a larger number, that is to say, it becomes the nth power form of a multiplied by 10, where 1 is greater than or equal to a greater than 10, and N is a positive integer?

For example, 2000 is 2 times 10 to the third power, n = 3

What are the polysyllabic characters in the list of new words in the second volume of sixth grade Chinese books
All in the new word list

[A1] ～ Yi ～ Hong ～ men [E1] ～ Jiao ～ Yan ～ Rou Mian [Ai1] ～ Ge Er ～ CI ～ nearly [AI2] ～ Da Ai [Ai1] ～ sigh [AI4] (exclamation) boil [AO1] ～ cabbage ～ tofu [AO2] ～ night fry ～ Ao 4 ～ mouth [niu4]

In the parallelogram, ab = 4, the coordinates of point D are (0,8), the parabola y = ax's square + BX + C with point C as the vertex passes through the point on the X axis as AB, (1) find a, B, C

The coordinate of point C is (4,8). Then there is - B / (2a) = 4, a * 4 ^ 2 + 4B + C = 8, and for Ax ^ 2 + BX + C = 0, there is (x1-x2) ^ 2 = 16, that is, (x1 + x2) ^ 2-4x1x2 = (B / a) ^ 2-4c / a = 16
A = - 2, B = 16, C = - 24
So the solution of ax ^ 2 + BX + C = 0 is X1 = 2, X2 = 6. So a (2,0), B (6,0)
Similarly, when the coordinate of point C is (- 4,8), there are a (- 2,0), B (- 6,0)
I don't know the exact coordinates of C. anyway, the answer is one of the two

1+5+9+13+17+``````+77=?
Hurry up,

The tolerance is 4
And the first term plus times the number of terms divided by 2
（1+77）*19/2

8g of the mixture of Cu and CuO was dissolved in 100ml of 98% concentrated H2SO4 with density of 1.84g/cm3, heated and stirred to make it completely dissolved, and 2.24l of gas was obtained under the standard condition. Assuming that the volume of the solution before and after the reaction had no obvious change, the solution was obtained
(1) Mass of reduced H2SO4 in the reaction
(2) Mass fraction of CuO in the original mixture
(3) The amount and concentration of solute in the solution after reaction

(1) There is no gas in the reaction of CuO and concentrated H2SO4, and the gas produced can only be SO2 from the reaction of Cu and concentrated H2SO4
By atomic conservation, n (SO2) = 0.1mol
That is to say, the reduced H2SO4 is 0.1mol
8 G
(2) From the reaction equation of Cu and concentrated H2SO4, 1CU ~ 1so2 can be known
So n (Cu) = 0.1mol, i.e. 6.4g
So m (CuO) = 1.6g
The mass fraction of CuO was 20%
(3) It is known from the above that m (CuO) = 1.6g, i.e. n (CuO) = 0.02mol
After reaction, the solute is CuSO4
Conservation by atoms n (Cu2 +) = 0.02 + 0.1 = 0.12mol
So the mass concentration of solute = 0.12 / 0.1 = 1.2mol/l

There is a point E on the BC side of the square ABCD, AE = 10cm. If the paper is folded along the vertical line of AE, so that the point e coincides with a, the length of Mn is requested

MN＝AE＝10NM
(prove triangle amn and triangle Abe congruence, AAS)

Simplify the following ratios and calculate the ratio. 6.4:4.8 4 / 2:5 4 / 35:3 / 21 4:1 / 2

6.4:4.8 = 64:48 = 4:3 ratio 4 / 3
4 / 2:5 = 2:5 ratio 2 / 5
4 / 35:3 / 21 = 4:5 ratio 4 / 5
4: 1 / 2 = 8:1 ratio 8

Calculating quadratic radical
√12-√27-√20+√50

√12-√27-√20+√50
=2√3-3√3-2√5+5√2
=5√2-2√5-√3

As shown in the figure, in the parallelogram ABCD, ab = 4, be is perpendicular to AC, and the perpendicular foot is e. extend be to intersect the extension lines of CD and ad at points F and g respectively
1. When point G coincides with point D, find the length? 2 of edge BC. When point F is the midpoint of CD, find the ratio? 2 of the area of triangle BCE and triangle ABG?

(1) ABCD is a parallelogram, BD ⊥ AC, ABCD is a diamond, BC = AB = 4. (2) ABCD is a parallelogram, ad ∥ BC, ab = CD, FBC = g, FCB = FDG, FC = FD, FBC ≌ FGD, BF = FG, ab ∥ CD, EAB ∥ ECF, be / EF = AE / CE = AB / CF = 2