1. The distance between a and B is 2.8 kilometers. A and B start from a and B at the same time and walk in opposite directions. A's speed is 12 kilometers per hour. B rides a motorcycle. After 6 minutes, they meet each other to find B's speed 2. Workers install a batch of machines, 4 sets per day, which are expected to be completed in several days. After two-thirds of the installation, they use the new method to install, and the work efficiency is increased to 1.5 times of the original. Therefore, they complete the task one day ahead of time. How many machines are there? How many days are they planned to complete

1. The distance between a and B is 2.8 kilometers. A and B start from a and B at the same time and walk in opposite directions. A's speed is 12 kilometers per hour. B rides a motorcycle. After 6 minutes, they meet each other to find B's speed 2. Workers install a batch of machines, 4 sets per day, which are expected to be completed in several days. After two-thirds of the installation, they use the new method to install, and the work efficiency is increased to 1.5 times of the original. Therefore, they complete the task one day ahead of time. How many machines are there? How many days are they planned to complete

The velocity X of 1 b
2.8/(12+x)=6/60
x=8
The machine has X
x/4-(x/4 *2/3 +x/3/(4*1.5))=1
x=36
x/4=9

1. If a commodity is sold at a fixed price, it will make a profit of 45 yuan per unit. Now, if you sell 8 units at a discount of 8.5% of the fixed price, you can get the same profit as if you sell 12 units at a discount of 35 yuan per unit?
2. In December of the calendar, the sum of the four dates of Zhang Yao's birthday is 80. Which day is her birthday?

1. If the purchase price is x yuan, then the price is (x + 45) yuan. From the meaning of the question, we can get [(x + 45) * 0.85-x] * 8 = (x + 45-35-x) * 12. If the purchase price is 155 yuan, then the price is 200 yuan
2. If the date of her birthday is set as x day, then the above day will not be the same, and the following day will be x + 7, X-7. If the left and right days are X-1, x + 1 respectively, then x + 7 + X + 1 + X-1 + X-7 + X = 80 can be solved to get x = 16, so her birthday is December 16 (in fact, after knowing the relationship between the above and below dates, you can directly list 5x = 80)

Given the function FX = a times x square - (a1 + 1) x + 1, when x = (- 1 / 2), the inequality FX is greater than 0, which is always true
How to calculate the value range of real number a

F (x) = a x ^ 2 - (a + 1) x + 1. When x = (- 1 / 2), the inequality f (x) is greater than 0
It's very simple. Put x = (- 1 / 2) into the column f (x) > 0 and solve a
F (- 1 / 2) = A / 4 + A + 1 + 1 = 5A / 4 + 2 > 0
5a/4+2>0
5a>-8
a>-8/5

Given that the equation of circle O is the square of X + the square of y = 16, find the tangent equation of circle O passing through point m (- 4,8)

Let the tangent be Y-8 = K (x + 4), that is, kx-y + 4K + 8 = 0
The distance from the center of the circle (0,0) to the straight line is radius 4
therefore
｜4k+8｜/√(1+k²)=4
(4k+8)²=16+16k²
16k²+64k+64=16+16k²
64k=-48
k=-3/4
y-8=-3/4(x+4)
y=-3/4x-3+8=-3/4x+5

When all electric appliances (total power 600W) of electric energy meter 220V 10A 3000r / kWh work for 4 hours, how many turns does the turntable turn

The total work consumed is 600W x 4H = 2400wh = 2.4kwh
Revolution = 3000 x 2.4 = 7200r

I'm too lazy to count myself
Xiao Ming wants to buy a 500 yuan mobile phone. His monthly allowance is 52 yuan. How long does he have to save to buy a mobile phone

9 * 52 = 468 in 9 months
10 * 52 = 520 in 10 months
So it's 10 months

An electric kettle with rated voltage of 220 V, rated power of 1000 W, frequency of 50 Hz, net weight of 0.6 kg and capacity of 1.2 L. what is the normal working current and resistance of the electric kettle?

1 degree electricity per hour: 1.6 degree electricity for 1.2L water without heating

1/（X+1)(X+2）+1/(X+2))(X+3)+...+1/(X+2005)(X+2006)=1/2X+4012

1/（X+1)(X+2）+1/(X+2))(X+3)+...+1/(X+2005)(X+2006)=1/2X+4012 1/(x+1)-1/(x+2)+1/(x+2)-1/(x+3)+...+1/(x+2005)-1/(x+2006)=1/2(x+2006)1/(x+1)-1/(x+2006)=(1/2)*1/(x+2006)1/(x+1)=(3/2)*1/(x+2006)3(x+1)=2(x...

There is an electric fan with rated working voltage of 220 V, rated electric power of 50 W, coil resistance of 0.4 Ω. The electric fan is connected to the circuit with voltage of 220 v. four students have calculated the heat generated per minute as follows, and the correct one is ()
A. Q=Pt=50×60=3000JB. Q=IUt=50×220×60220J=3000JC. Q=I2Rt=（50220）2×0.4×60J=1.2JD. Q=U2tR=2202×600.4J=7.26×106J

The heat generated during the normal operation of the electric fan is due to the internal resistance heating, so the heat generated per minute is q = i2rt = (50 / 220) 2 × 0.4 × 60j = 1.2J, so C is selected

The focus F1, F2 and point P of the ellipse x ^ 2 / 12 + y ^ 2 / 3 = 1 are on the ellipse. If the midpoint of the line segment Pf1 is on the y-axis, how many times of PF is PF2?

a^2=12,b^2=3,c^2=12-3=9
So F1 (- 3,0), F2 (3,0)
If the midpoint of Pf1 is on the y-axis, then the abscissa of P is 3, and 9 / 12 + y ^ 2 / 3 = 1 is substituted into the elliptic equation
y^2=3/4
Y = (+ / -) radical 3 / 2
That is, Pf1 = root [(3 + 3) ^ 2 + 3 / 4] = root 147 / 4 = 7 / 2 root 3
PF2 = 2a-pf1 = 4 radical 3-7 / 2 radical 3 = radical 3 / 2
So, Pf1 / PF2 = 7
That's seven times