Please use the law of voltage and current in parallel circuit and Ohm's law to deduce the resistance 1 / r = 1 / R1 + 1 / R2 (proof)

Please use the law of voltage and current in parallel circuit and Ohm's law to deduce the resistance 1 / r = 1 / R1 + 1 / R2 (proof)

U1 = U2 = u in parallel circuit
I1=U/R1
I2=U/R2
I=I1+I2=U/R
After making an appointment

How to factorize cubic equation of one variable
After reading the explanation on the Internet, I didn't understand it. I'll give you a problem. I'll give you a solution
x^3 - 2x^2 - 19x + 20
If it is convenient, there is also a quartic equation as follows:
x^4 + 11x^3 +38x^2 +40x
The process doesn't matter. I just want to know where to start,
Of course, the more detailed the better

High order equations in middle school can be decomposed simply,
First try some simple integer roots, such as - 1,0,1, etc
For example, if x ^ 3-2x ^ 2-19x + 20, the sum of coefficients is 0, it means that there is a factor X-1,
Then x ^ 3 - 2x ^ 2 - 19x + 20 = (x-1) (x ^ 2 + ax + b), the expansion comparison coefficient is A-1 = - 2, - 1 * b = 20,
So x ^ 3 - 2x ^ 2 - 19x + 20 = (x-1) (x ^ 2-x-20),
Finally, x ^ 2-x + 20 = (x + 4) (X-5) is decomposed by cross multiplication
Similarly, we can decompose x ^ 4 + 11x ^ 3 + 38x ^ 2 + 40x = x (x + 2) (x + 4) (x + 5)

China's 21:30 on December 1, 2012 is Britain's

13:30, December 1, 2012

A pile of coal, originally planned to burn 450 kg per day, burned out in 40 days, actually burned out in 30 days, how many kg more per day than originally planned?
Xinxing Electric appliance store sells color TV sets. In the morning, more than half of the total number of color TV sets are sold, and in the afternoon, more than half of the remaining 20 sets are sold, and there are 95 sets left. How many original color TV sets are there?

450*40/30-450=150
[(95+20)*2+10]*2=480

Shanghai World Expo will be held from May 1st to October 31st, 2010. May 1st is Saturday, so what day is October 31st?
Give the formula, don't come if you don't give it.

It's Thursday
Because it took 152 days
152÷7=21.5
7-5=2
Two days ahead of Saturday is Thursday
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More important formula, such as Pythagorean theorem, etc

For example, the following two are squares
a2+b2=(a+b)(a+b)
a2-b2=(a+b)(a-b)
(a + b) 2 = A2 + B2 + 2Ab (2Ab is 2, not square)
(a-b) 2 = A2 + b2-2ab (ditto)
These are more important, I am the third day, these to the third day is also to use

Simple calculation of 5 / 7 × 57 / 8

5/7×57/8
=5/7×（7+1/8）
=5/7×7+5/7×1/8
=5+5/56
=5 and 5 / 56

In the formula of work efficiency = total amount divided by time, why can the total amount be 1?
I don't understand when solving equations

Work efficiency = total amount of work △ working hours
Why can the total amount be 1?
For example, if a project is completed by team a in 6 days, then the work efficiency of team a is 1 / 6

5.2 divided by 3 and 1 / 5 and 2 / 3 minus 1 times 0.7?

First multiplication and division, addition and subtraction. I think 11 / 24

Xiao Huang took part in a meeting. Xiao Zhang, Xiao Niu, Xiao Wang and Xiao Li shook hands with each other to show their friendship. Xiao Zhang shook hands three times, Xiao Niu four times, Xiao Wang two times and Xiao Li one time. How many times did Xiao Huang shake hands

Hold calf and Zhang twice
Xiao Niu holds it four times, which means everyone has held it. Xiao Li holds it only once with Xiao Niu. Xiao Zhang holds it three times, which means everyone except Xiao Li has held it