I m going to ask a few math questions, and I hope those who are interested will come in and take a l

Don't you know the answer?

I think you have the answer!

I didn't write the process on the answer!

If you encounter a problem, you have to think for yourself.

Little P child doesn't study well, run this cheating? I can't tell you! Isn't this a waste of the future flowers of the motherland! Kids, study hard! Don't think about crookedness! If you were my son, I would kill you with a pot of! Your family paid for you to come here to find answers?

I don't know if you've learned to solve unknowns? If not, I don't know what's going on.

Lemma: H is an acute triangle ABC stool opening the center, and the radius of ABC circumscribed circle is R, then AH=2RCOSA

Even 0c intersects o at the point g, and ag, bg

GB Vertical BC

gb//ah

Angular bhd = angular abe + angular bad

90-angular bac + angular bad

90-angular BGC + angular bad

Angular bcg + angular bad

Corner bag + corner bad

Horn Gadag BH

AGHB is a parallelogram.

ah=gb2rcosgbc

2rcosa

Proof: Angular AFC = Angular AEC = 90

a, f, c, e are all round, and ac is the diameter of its jujube.

ef=2rsineaf=ac*sineaf is known by lemma: ah=2rcoseaf=ac*coseafef 2+ah 2=ac 2

ah=√(a^2-b^2)

1. If the number of fixed points of a polygon is doubled, and its internal angle is 2520°, then the number of vertices of the original polygon is (c)a, 6 b, 7 c, 8 d, and 9

2. Except for an inner angle of a polygon, the sum of the remaining inner angles is 2570°, then the degree of this angle is (d).

There is a difficult point in this question, that is, which side of the midpoint the two meet, that is, who is faster, which is faster in A and B, which is not given as a known condition in the question, so that the solution is not available. Careful analysis, they walked the whole distance in about 100 minutes, according to the speed of normal people should be around 8000 10000m, to the midpoint near the distance of the two people walked is only 2 * 2 = 4m, the complete distance is only 8 m difference, this distance is only about 1 1000 in the whole process, ignoring this distance can only cause an error of 1 1000 orders of magnitude, for the result of nearly 100 minutes, the error of these seconds is acceptable. For this reason, ignoring the 2m of "meeting at a distance of 2m from the midpoint", looking directly at the meeting at the midpoint, not caring about the speed error of about 1 1000 between the two people, and looking at them at equal speed, this will greatly simplify the problem and change the unsolved to the solution.

After the two meet at the midpoint, walk for another 5 minutes, B meets C, and A also walks out for 5 minutes, so the distance between A and C at this time is the distance of A walking 2*5=10 minutes, but A still needs 45 minutes to reach the end point, then the distance from the meeting place of B and C to the end point A walked 45 + 10 = 55 minutes, and C walked for 45 minutes, and the speed ratio of the two is 45 55

A walked 5 + 45 = 50 minutes from the middle point to the finish line, and the whole journey was 50 * 2 = 100 minutes. Then C took 100 * 45 55 = minutes to walk the whole way, and he left after A's departure, that is, 18 minutes and 11 seconds.

∵-1＜x＜3

k 0, x = -1, x = 3 should be the solution of the equation kx -(k +1) x-3 = 0 into the calculation k=1

k = 1 is actually -1 and 3 are the two solutions of the inequality into the equation, and you can find it by bringing it in.

When the constructor f(x)=kx 2-(k 2+1)x-3k=0, f(x)=-x-3

The solution set satisfying f(x)<0 is x>-3

So k≠0 if k<0

The image opening of f(x) is downward, and the solution set satisfying f(x)<0 is inconsistent with the known solution set from a certain point to negative infinity and from a certain point to positive infinity.

So k>0

f(x) has an image opening pointing upwards.

And there are two intersections with the x-axis (-1,0), (3,0)f(-1)=k+k2+1-3=0

k^2+k-2=0

k-1)(k+2)=0

k = 1 or k = -2 (rounded).

f(3)=9k-3(k^2+1)-3=0

3k^2-9k+6=0

k^2-3k+2=0

k-1)(k-2)=0

k = 1 or k = 2

In summary, k=1

This is the number of combinations of 3 different elements from 15 different elements c(15,3)=15*14*13 6=455

The first one is believable, but you may not understand it. Otherwise, I wouldn't have asked the question. It can be analyzed like this.

1。What is 1; There are several combinations.

1，2，3。1,2,4 1,2,5 A total of 13 species.

1,3,4 1,3,5 1,3,6 1,3,7 There are 12 combinations.

1,4,5, 1,4,6 1,4,7 A total of 11 species.

2。Does not contain 1:

It can be seen that in the combination of 3 numbers ("A", "B", "C"), A has 15 cases, you can take 1 of the 15 to have 15 choices, B can find a certain number from any of the remaining 14, there are 14 different choices, C can find a certain number from any of the remaining 13, and there are 13 different choices. So there are 15*14*13=2730 combinations, and there are 3,1,2 and 2,3,1, 2,1,3 are the same case each, so put 2730 2*3=455