In a sinusoidal AC circuit, the frequency of voltage and current is the same, but the initial phase is not necessarily the same. Why? Give an example!

It is very common that the phase of voltage and current at the same frequency is different according to the nature of the load. For example, in pure resistance circuit, the phase of voltage and current is the same. In inductance and capacitance circuit, because the current and voltage can not change suddenly, the phase is also lead and lag

If the frequency of a sinusoidal signal is known, how to measure an unknown frequency?

See how many times their wavelength or period difference, the frequency of wave growth is small, and so on

The frequency of output sine AC signal of function generator is 20kHz. Can we measure it with AC voltage of digital multimeter

Yes, the average value measured by the AC meter. Just pay attention to the voltage range. However, due to the existence of high frequency, the capacitance effect is relatively obvious, and the error has not been specifically observed, so it should be relatively small, so it can be said
It is suggested to use oscillograph to see the waveform and calculate the result

The square of 7x + 13x-8 = 0

The author needs to understand that "cross multiplication" can not be solved by all quadratic equations of one variable. This method only works for some individual problems. Therefore, this method has been deleted in books, because it can only be used for some special equations. However, as a junior high school student, we must master "cross multiplication"

One fourth of the male students is 50 more than one third of the female students. Three fourths of the male students are twice as many as the female students. How many male and female students are there in this school?
Do not use equations

According to the known column arithmetic formula:
1 / 4 male = 1 / 3 female + 50; 3 / 4 male = 2 female;
From this we can see that the three times formula 1 equals formula 2, that is, 3 × 1 / 4 male = 3 × (1 / 3 female + 50) = 3 / 4 male = 2 female;
After sorting out: female + 150 = 2 female, the number of female students is 150;
According to the number of girls, the number of boys is 400

It is known that f (x) = 2ax-b / X INX has extremum at x = - 1 and x = 1 / 2. Can X be taken as - 1?

According to the definition of logarithm, we know that the true number is greater than zero, that is, the default condition of the title is x > 0, so we should give up one of the results. We can't take - 1

There are 540 students in the sixth grade of experimental primary school, of which the number of girls is 80% of the number of boys. How many boys and girls are there in this grade?

540 (1 + 80%), = 540 △ 1.8, = 300 (people); 540-300 = 240 (people); a: there are 300 boys and 240 girls in this grade

Find the value of X in the following problem: the square roots of 2x-1 are A-3 and 3a-1

According to the meaning of the title
a-3+3a-1=0
∴a=1
∴2x-1=(a-3)²=4
x=5/2

The sum of a, B and C is 25, the sum of a and B is 5 larger than C, and B is 3 smaller than C. If a is x, B is y, C is Z, the equations are established

The equations are: x + y + Z = 25
x+y--z=5
z--y=3

If the opposite number of a is non negative, then the relationship between a and 0 is a______ 0．

According to the meaning of the question, - a ≥ 0, a ≤ 0

In a sinusoidal AC circuit, the frequency of voltage and current is the same, but the initial phase is not necessarily the same. Why? Give an example!