Solve, please write the reason for each step, thank you

1 Let's use a simple indefinite equation first.

Set the right x channel, wrong y, and the one that didn't answer was 25-x-y

Failure to answer has no effect on the score here, but it must be met that 25-x-y is an odd number.

2x-y=34 It can be seen that x is not less than 17, but it is not more than 25 x+y is not greater than 25

Use the enumerative method to solve.

x y17 0 (not suitable).

19 4 (not suitable).

20 6 (not suitable).

So that's right, 18 ways.

2. The area is 8 times more than the original, so the area is 9 times the original.

That is to say, the radius is 3 times the original, the circumference is 3 times the original, and 2 times more than the original is centimeter with centimeters) is the diameter of the circle, and the area is.

radius 4 squared = square centimeters).

3. You may wish to set these two iron rods into a and b, then a is 2 3 b 4 5, a 4 5b 2 3 6 5b

18-15 3 Answer: The difference between the first iron rod and the second iron rod is 3

Let the correct answers, the unanswered, and the incorrect answers be x y z

Then there is 2x-z=34 x+y+z=25 to get 3x+y=59 and y is known to be odd, so only x=18 y=5 z=2 matches the answer

The question you ask is simple.

First of all, a cabbage can be exchanged for x radishes, and the little gray rabbit can be exchanged for a.

In this way, the little white rabbit has a cabbage and 200-ax turnips, while the little gray rabbit has 80-a cabbage and ax turnips.

To make the quantity of grain equal, there is 80-a=a ax=200-ax, and a gives 40 x=

That is, 2 cabbages for 5 turnips.

I didn't have time to finish answering last time, I'm sorry.

It is stipulated that the answer to X questions, the wrong answer to the Y question, the unanswered question, the Z unanswered question is an odd number and 2x-y=34, you can get Y is an even number, X is an even number, Xiao Ming scored a total of 34 points, at least 17 questions were answered correctly, and 18 were directly calculated by the number method

The area is 9 times larger (8 times more than the original).

That is to say, the radius is 3 times the original, the circumference is also 3 times the original, and 2 times more than the original is centimeter with times centimeter) is the diameter of the circle, and the area is.

radius 4 squared = square centimeters).

It is advisable to divide these two iron rods into a and b, then a is 2 3 b 4 5, a 4 5b 2 3 6 5b

18-15 3 Answer: The difference between the first iron rod and the second iron rod is 3

Gear circumference:

6 cm) pinion circumference:

3 cm) The big gear turned 4 weeks and went:

cm) pinion to turn: week).

I think it's more practical than this when you look at the textbook.

It's all conceptual questions.

x 2 + y 2-2x-4y=0 is resolved to obtain (x-1) 2+(y-2) 2=5 to obtain the center of the circle (1,2).

The straight line is bisected by the circle side, so the straight line is staring at the circle banquet, and the straight line is y-2=k(x-1).

Solution: Set the purchase price of each table is X yuan, then Huai Li Yin will increase the pricing of 40 yuan according to the purchase price is (1 + 40) X yuan, and the price will be discounted by 90 yuan, and the income after the delivery of 58 yuan will be [yuan, and the profit of each table will be [yuan; According to the meaning of the question, the equation can be listed:

x=1000

1. This function is an odd function, then f(0)=0 and f(1)=f(1), get:

b 1) (a 2) = 0 and [b (1 2)] (a 1) = [b 2] [a 4].

obtained: a=2, b=1

So, f(x)=[ 2 x 1] [2 (x 1) 2]=[1 2 x] [1 2 x].

2. f(x)=[1 2 x] [1 2 x]=(1) 2 (2 x 1), i.e., f(x) decreases on r.

Again: f(t 2t) f(2t k)<0

f(t²－2t)<－f(2t²－k)

f(t²－2t)－2t²＋k

The minimum value of k<3t 2t [k< [3t 2t] is sufficient, and the minimum value is 1 3].

k<－1/3

z=x 3 *y -x 2*y 2, then find the partial derivative of x to get z'x= 3x 2 *y -2xy 2 to find the partial derivative of y to get z'y=x 3 -2x 2 y, then find the second-order partial derivative to get z''xx=6xy -2y^2z''xy=3x^2 - 4xyz''yy= -2y^2