Math practice questions: Junior high school 2 urgent, urgent, urgent

1.After connecting a 50 resistor in series in a certain circuit, the current in the circuit becomes half of the original, then the resistance in the original circuit is 50 ohms If you want to make the current in the circuit become the original times, you need to connect a resistor in parallel in the circuit with a resistance value of 2 times the original.

2.（1）20mhz=___20000___khz=__20000000___hz。（2）3600000hz=__。3）984000khz=__984___hz=__。

3.A and B electromagnetic waves, the wavelength of A is 30 times that of B, then the frequency of B is 30 times that of A, and the ratio of the wave velocity of A and B electromagnetic waves is 1

Supplements: 4(1) The radio signal is transmitted to the communication satellite at high altitude, and the TV station receives the signal transmitted back by the satellite, and there is a large transmission distance in the middle, and the signal transmission distance transmitted by the cable in the studio is small, and the transmission speed is the same, and the satellite signal takes a long time, so it causes a time delay.

2）h=36000km=

v (light) = 3 * 10 8m s

t = (2h) v (light) = (2*

Because the transmission distance of the signal from the cable in the studio is small, much smaller than the transmission distance of the satellite signal, the time is negligible.

So the time difference is.

The angular DBC is 30° by proving that DAE and DBE are congruent and the second CD is similar to ADE and ABC.

v = π(r² -r²)h==

847,800 cubic centimeters.

Cubic metre. The two significant figures are cubic meters.

Solution: According to the meaning of the question, get.

Concrete required 75 2 45 2 300 300 (75 45) (75 45) 300 3600

3,391,200 (cubic centimeters).

cubic meters) Answer: (omitted).

r = ，r = ，h = 3m

v = π(r² -r²)h==

847,800 cubic centimeters.

Cubic metre. Cubic metre.

Yes === 847800 cubic centimeters.

Cubic metre.

Solution: r = , r = , h = 3m

v = π(r² -r²)h

h(r + r) (r- r)

Bring your own counts!

I wish you progress in your studies!!

Little brother, you really have to study hard.

The idea of solving this problem is the outer volume - the inner volume.

First of all, if you look at it as solid, that's *r*r*l (r is the outer radius), which is cubic centimeters.

This is hollow, and the inner volume is *r*r*l (r is the inner radius), which is the cubic centimeter.

In the end, we want the actual volume to be two subtracted cubic centimeters = cubic meters.

Directly into rice units.

Let's just forget about it.

The idea is that the volume of the cylinder calculated with the outer diameter is subtracted from the volume of the cylinder calculated with the inner diameter.

Excuse me.. The inner diameter seems to refer to the diameter, so it is all divided by two and then calculated.

(75/2^2π-45/2^2π)×300=(75/2^2-45/2^2)π×300=（

There may be a miscalculation, so it's best to check it.

If you don't want a process, assume x=1, y=1, z=1It is to take a special value. Bring in. Get 9

If it's a short-answer question... I haven't figured it out yet.