Why is a nonempty subset obtained in this way = 2 ^ n-1? (n represents the number of subsets)

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• 1. A problem of number axis and linear equation Find out the linear equation of triangle area 12 which is the intersection of two lines x-2y + 8 = 0 and 3x + 2Y = 0 and cut with the negative half axis of X axis and positive half axis of Y axis
• 2. How much is 4.3 plus 3.7?
• 3. How to solve the equation 4 / 1 = Y-10 / x + 8 - y + 32 / x + 18 The column above is wrong -1. Wrong The original problem is: a fraction, such as numerator plus 8, denominator minus 10, the simplified value is equal to 4 / 3; such as numerator plus 18, denominator plus 32, the simplified ﹛ x-10/y+8=4/3 The value after is equal to 2 / 1, and this fraction is (hint: let the fraction be x / Y {to solve this equation ﹛ x+32/y+18=2/1
• 4. 1. Find out the angle a (0 ° ≤ a < 360 °) where the end edges of the following angles coincide, and judge which quadrant the following angles are （1）-1441°（2）1790° 2. Fill in the blanks (1) The set of the first quadrant angles is: (2) The set of angles of the end edge on the coordinate axis is:
• 5. How to solve the equation (X-2) * 75 = (x + 5) * 60
• 6. Mathematical calculation problem: (- 1 and 1 / 2) - 1 and 1 / 4 + (- 2 and 1 / 2) - (- 3 and 3 / 4) - (- 1 and 1 / 4) + 4 =? |-2 and 1 / 2 | + (- 3.7) + | - (+ 2.7) | - - (7 and 1 / 2) | =?
• 7. In triangle ABC, the area of triangle ABC is 90, C = 5cm, a: B = 2:3?
• 8. In RT angle ABC, the opposite sides of ∠ C = 90 ∠ a ∠ B ∠ C are a, B, C (1) if a = 3cm, B = 5cm, then C = (2) if a is more than B = 3 than 4, and C = 10cm, then a = B As long as the answer is no picture
• 9. In △ ABC, ∠ A: B: C = 1:1:2, then ∠ C = 90 ° if CD is an angular bisector and CD = 5cm, then ab=
• 10. If AC = 2.4cm, BC = 1.5cm, then the area of △ AEC is______ ．
• 11. The intersection coordinates of the inverse scale function and the positive scale function If there is an intersection (m, n) between the positive scale function y + K1X (K1 ≠ 0) and the inverse scale function y = K2 / X (x ≠ 0), then the coordinates of the other intersection are (,) 2. In the same coordinate system, make the image of function y = 2 / X and function y = X-1, y = X-1, and use the image to find their intersection coordinates
• 12. Simple algorithm of 36 + 18 × 37 + 19 × 36
• 13. Find the image of function y = x square + 1 / X (x is not equal to 0) Teach me the method
• 14. When m is the value, the quadratic equation MX2 + (m-1) x + M = 0 (1) If the solution set of the inequality MX2 + (m-1) x + m > 0 is r, the value range of M is obtained I think there is something wrong with the title.
• 15. Two trains, passenger cars 200 meters long, 300 meters per second; freight cars 300 meters long, 20 meters per line, two cars in parallel track up into a few seconds later, passenger cars over freight cars
• 16. A passenger car and a train run in the same direction on parallel tracks. The length of a passenger car is 200 meters, and that of a freight car is 280 meters The speed of the bus is 1200 meters per second, and the speed of the train is 720 meters per second. How many seconds is the crossing time of the two trains? (solution of equation)
• 17. A gymnasium inlays the floor with rectangular wood blocks of the same size, and paves two blocks for the first time, as shown in Figure 1. The second time completely encloses the first time, as shown in Figure 2 and figure 3 The second paving is completely enclosed, as shown in Figure 3. In this way, after the nth paving, the letter N is used to indicate the number of wood blocks used in the nth figure
• 18. Figure 1 is 9cm in length and 6cm in width, figure 2 is 3cm in length and figure 3 is 12cm in width
• 19. As shown in the figure, it is known that ∠ 1 + 2 = 180 ° and ∠ 3 = B. try to judge the relationship between ∠ AED and ∠ C, and rationalize the conclusion
• 20. Send a 5-inch, 5.3-inch, 5.5-inch, 6-inch mobile phone comparison chart? I really don't know what size is