A story book costs 18.5 yuan, and the price of a science and technology book is twice that of a story book, less than 1.8 yuan?

A story book costs 18.5 yuan, and the price of a science and technology book is twice that of a story book, less than 1.8 yuan?

The price of a science book is 18.5 times 2-1.8 = 35.2
The two books add up to 35.2 + 18.5 = 53.7

The school paid 882 yuan for 156 books. The price of a story book is 1.5 yuan cheaper than that of two science and technology books. The unit price of science and technology books and comic books is 4.2 yuan, and the price of science and technology books is 1.5 yuan
How many of the three books do you buy?

Storybook: 4.2 × 2-1.5 = 6.9 yuan
There are x comic books, 2x science books and 156-3x story books
( 2x+x)×4.2+（156-3x)×6.9=882
12.6x+1076.4-20.7x=882
x=24
There are 24 comic books, 48 science books and 84 story books
Thank you

When calculating the total power consumption, can the power of 380 V and 220 V be added directly?
With 380 V equipment, the power is 20 kW. With 220 V equipment, the power is 10 kW
So is the total power consumption 30 kW?

Can be added directly! Power and voltage level does not matter

High school sequence known A1 = 1, a (n + 1) = 3an + n & # 178;, find an
The second problem is that the square of an = a * a (n + 1), where a1 = 1, an > 0, a > 0, find an

Question 1: from A1 = 1, a (n + 1) = 3an + N, we get that:
[a(n+1)+(1/2)*(n+1)^2+(1/2)*(n+1)+1/2]/[an+(1/2)*(n^2)+(1/2)*n+1/2]=3
So [an + (1 / 2) * (n ^ 2) + (1 / 2) * n + 1 / 2] sequence takes (a1 + 1 / 2 + 1 / 2 + 1 / 2) as the first term, and 3 is the equal ratio sequence of common ratio
That is: an + (1 / 2) * (n ^ 2) + (1 / 2) * n + 1 / 2 = (a1 + 1 / 2 + 1 / 2 + 1 / 2) * 3 ^ (n-1) = (5 / 2) * 3 ^ (n-1)
So an = (5 / 2) * 3 ^ (n-1) - (1 / 2) * (n ^ 2) - (1 / 2) * n-1 / 2
The second question: from A1 = 1, an ＞ 0, a ＞ 0, the logarithm of an ^ 2 = a * a (n + 1) is obtained
2ln(an)=lna+ln[a（n+1）]
So {ln [a (n + 1)] - LNA} / {ln (an) - LNA} = 2
So the {ln (an) - LNA} sequence is an equal ratio sequence with (LN (A1) - LNA) as the first term and 2 as the common ratio
That is: ln (an) - LNA = (LN (A1) - LNA) * 2 ^ (n-1)
So ln (an) = [1-2 ^ (n-1)] LNA
So an = a ^ [1-2 ^ (n-1)]

Given the mass of salt, the mass of water, the volume of water, how to calculate the volume brine density?
The mass of salt is 19g, the volume of water is 60cm, the volume of brine is 70cm, what is the density of brine?

If it is known that the mass of water is m water, the volume of water is v water, the mass of salt is m salt, and the volume of salt is v salt, then the formula for calculating the density of salt water at a certain temperature is: the mass of salt water solution / the volume of salt water solution = the density of salt water (the mass of salt water per unit volume), that is: (M Salt + m water) / (V Salt + V water) = g / ml

______ It's called the first level operation______ It's called the second level operation

According to the definition of four operations, addition and subtraction are called first level operations, multiplication and division are called second level operations

A student uses a voltmeter with a range of 0-3v and 0-15v to measure three used dry batteries. If the voltage is 13V, what is the actual voltage? Why?
Yes, it's tandem

Because the voltage of each dry cell is 1.5V, and the voltage after series connection is 4.5V
The voltage of each battery is 2V and 6V after series connection
The battery voltage remains constant at all times
So, a classmate
1. The range of voltmeter is inaccurate
2. His battery is not any of the above
3. He read the wrong number

1/3+2/8+2/15+2/24+2/35+2/48+2/63+2/80

Well, the original formula = 2 (1 / 8 + 1 / 24 + 1 / 48 + 1 / 80) + 2 (1 / 15 + 1 / 35 + 1 / 63) + 1 / 3 = 2 [1 / (2 * 4) + 1 / (4 * 6) + 1 / (6 * 8) + 1 / (8 * 10)] + 2 [1 / (3 * 5) + 1 / (5 * 7) + 1 / (7 * 9)] + 1 / 3 = [1 / 2-1 / 4 + 1 / 4-1 / 6 + 1 / 6-1 / 8 + 1 / 8-1 / 10] + [1 / 3-1 /

Solving four calculation problems
It includes four mixed operations, the power, the equation, the four trace,
1L is very good, but I feel I can't understand it, or I can't understand it, or I feel dizzy... For example, there is no satisfactory answer in the remaining two weeks.
I'll take you. I'll give you another 20 points

1.3 / 7 × 49 / 9 - 4 / 3 2.8 / 9 × 15 / 36 + 1 / 27 3.12 × 5

It is known that the hyperbolic equation is X-Y = 1, and there is only one common point between the line L passing through P (0,1) and the hyperbola, then the number of lines L is

y=kx+1
x2-y2=1
（1-k2）x2-2kx-2=0
When the coefficient of quadratic term is 0, it is parallel to the asymptote k = ± 1.2
When △ = 0, the tangent k = ± √ 2
So there are four