As shown in the figure, the quadrilateral ABCD is inscribed at the center O, CD is parallel to AB, and ab is the diameter of the center O, AE is perpendicular to the extension line of CD at point E, proving that AE is tangent to the circle o

AE vertical CD, CD / / AB = > AE vertical AB, where AB is the diameter of circle O and point a is on the circle = > AE is tangent to circle o

Why is the speed of electromagnetic wave equal to the speed of light?

First of all, light has wave particle duality, which is also a kind of electromagnetic wave
The velocity of electromagnetic wave in vacuum is equal to that of light in vacuum;
In the same medium, the longer the wavelength is, the smaller the refractive index is, and the higher the propagation speed is

There are two kinds of mobile phone calculation methods: globetron, charge 0.4 yuan per cent, 50 yuan per month. If Shenzhouxing is used, there is no monthly rent, and the call fee is 0.6 yuan per cent
(1) How much does it cost to make a local call within a month from 200 to 300?
(2) For a local call time, will the two billing methods charge the same?
(3) Mr. Zhang wants to get a phone card. Please give him some advice
It's math, not reality

A:
(1) The time is set as X minutes, GLOBCOM = 50 + 0.4 * x, China Travel = 0.6 * X
(2) Yes, when 50 + 0.4 * x = 0.6 * x, x = 250 minutes
(3) It is suggested that from (2), it is cost-effective for Mr. Zhang to choose China travel when the number of minutes is less than 250 minutes, and it is cost-effective for Mr. Zhang to choose global communication when the number of minutes is more than 250 minutes

As shown in the figure, in the known square ABCD, M is the midpoint of AB, e is a point on the extension line of AB, Mn is perpendicular to DM, and the bisector of the intersection angle CBE is at n, so MD = Mn
No, I'm sorry.

Certification:
Extend DB to point F, make BF = BN, connect MF
Then ∠ DBN = 45 ° + 45 ° = 90 °, and ∠ MBF = ∠ MBN = 135 °
∵MB=MB
∴△MBF≌△MBN
∴∠N=∠F,MN=MF
∵MN=MD
∴MF=MD
∴∠F=∠MDF
∴∠MDN=∠N
Let the intersection of BD and Mn be o
∵∠DOM=∠BON
∴∠DMN=∠DBN=90°
That is MD ⊥ Mn

There are 160 g of water with density of 1 g / cm cubic and alcohol with density of 0.8/cm cubic. Using them to prepare disinfectant alcohol with density of 0.84 g / cm cubic, the most effective way is to use water with density of 1 g / cm cubic and alcohol with density of 0.8/cm cubic
Continue above: how much can be prepared at most? (volume change during mixing is ignored)

210g 250ml

The taxi charging standard of a city is as follows: Li Li takes a taxi from home to her grandmother's home, paying a total of 11.2 yuan
The charging standards of taxis in a city are as follows:
Mileage charge
RMB 8.00 for 3km and within 3km
More than 3 kilometers, one-way, 1.60 yuan per kilometer
More than 3 km, round trip, 1.20 yuan for every 1 km
(1) Li Li takes a taxi from home to her grandmother's house, paying a total of 11.2 yuan. How many kilometers is the distance between Li Li's house and her grandmother's house? (2) Mr. Wang goes to the Personnel Bureau 6 kilometers away from the school to get a piece of information and immediately returns to the school. How can he get a better ride? How much is the taxi fee?

1. (11.2-8) / 1.6 + 3 = 5km
2 options one way (8 + 1.6x3) x2 = 25.6 yuan
Choose 8 + 1.2x9 = 18.8 yuan
It's cost-effective to choose a round trip!

If the length of a rectangle is reduced by 5cm, the area will be reduced by 20cm. If the width is increased by 3cm, the area will be increased by 30cm?

Let the original rectangle be x cm in length and Y cm in width,
According to the meaning of the title:
(X-5)Y=XY-20
X(Y+3)=XY+30,
The results are as follows
-5Y=-20,
3X=30
The solution is: x = 10, y = 4,
S rectangle = 10 × 4 = 40 cm and 178

In △ ABC, the opposite sides of angles a, B and C are a, B and C respectively. A = 3 is known, cos [(a + C) / 2] = (root 3) / 3. (I) the value range of B (II) the value range of vector AB * in △ ABC, the opposite sides of angles a, B and C are a, B and C respectively. A = 3 is known, cos [(a + C) / 2] = (root 3) / 3. (I) the value range of B (II) the value range of vector AB * and vector AC

In isosceles RT △ ABC, ∠ ABC = 90 ° AB = AC

Zhejiang children's Publishing House publishes the optimized edition of "one practice for each class of primary school mathematics in Grade 6" on page p88. Use your brain. The formula and answer of the first question! Come on,
It's a graph

Er... Don't you know? I'll find you again. I won't do it!
I only know which question below
45 in class A, 60 in class a!

If you draw the largest square in a circle, find the ratio of the area of the circle to that of the square
RT

The ratio of the area of a circle to that of a square = 157:100 suppose the radius of a circle = 1, the area of a circle = 1 × 3.14 = 3.14, the area of a square = (1 + 1) △ 2 = 1, the area of a circle: the area of a square = 3.14:1 = 157:100

As shown in the figure, the quadrilateral ABCD is inscribed at the center O, CD is parallel to AB, and ab is the diameter of the center O, AE is perpendicular to the extension line of CD at point E, proving that AE is tangent to the circle o