40 2／3•39 1／3=?

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• 1. 2/1+(3/1+3/2)+(4/1+4/2+4/3)+.+(1/40+2/40+...+38/40+39/40) One or two steps in the process, and the answer
• 2. 1/2＋（1/3＋2/3）＋（1/4＋2/4＋3/4)＋.＋（1/40＋2/40＋.＋38/40＋39/40） 1/2＋（1/3＋2/3）＋（1/4＋2/4＋3/4）.＋（1/60＋2/60＋.＋58/60＋59/60）
• 3. 2\1+(3\1+3\2)+(4\1+4\2+4\3)+.+(40\1+40\2+.+40\38+40\39)
• 4. 1 / 2 + (1 / 3 + 2 / 3) + (1 / 4 + 2 / 4 + 3 / 4) +. + (1 / 40 + 2 / 40 +... + 38 / 40 + 39 / 40) It can be seen that the number equidistant from the first place in each bracket is added, so the items in brackets can be arranged in reverse order and added Who can help me
• 5. 16 + 27 + 61 + 62 + 51 + 68 + 69 = (simple method)
• 6. Who is the third power of 2 or the second power of 3, the sixth power of 5 and the fifth power of 6, the fourth power of 3 and the third power of 4, the seventh power of 6 and the sixth power of 7
• 7. Decomposition factors: (1) the second power of 6x, the second power of Y, the third power of z-9xy, (2) the second power of 9a-6ab + 3A (3) - 7ab-14abx + 49aby (4) The second power of 4A B + the second power of 6ab-8a
• 8. If 2A + B = 0, then | A / | B | - 1 | + | a | / B-2 |, is equal to (). A, 2b, 3C, 4D,
• 9. There is a reaction a + 2B → 3C + 4D as follows. It is known that the sum of mass of a and B is 18G and the mass ratio of C to D is 4:5 How much is C
• 10. Given 8A ^ 3B ^ m divided by 28a ^ n of B ^ 2 = 2B ^ 2 of 7, find the value of M, n (2) If 10 ^ a = 20, 10b = 5 ^ - 1, find 9 ^ a divided by 3 ^ 2B
• 11. 1 × 2 / 1 + 2 × 3 / 1 + 3 × 4 / 1 + 4 × 5 / 1 +... + 39 × 40 / 1
• 12. My father is 47 years old and my son is 21 years old. How many years ago, my father was three times as old as my son?
• 13. My father is 32 years older than my son this year. Three years later, my father is five times older than my son. How old is my son this year
• 14. The father is 47 years old and the son is 21 years old. How many years ago was the father three times as old as the son? To solve the equation
• 15. My son is 13 years old. My father is 40 years old. Is there a year when my father is four times as old as his son?
• 16. Is there a year when the son is 13 and the father is 40? The father's age is 4 times of the son's? Why is it negative!
• 17. My son is 13 years old. My father is 40 years old. Is there a year when my father is four times as old as his son?
• 18. 1. My son is 13 years old and my father is 40 years old. Is there a year when my father's age is four times that of my son? Why? 2. Wang Lei went to the shoe store and spent 188 yuan to buy a pair of shoes. The shoes were sold after 20% off the price. The price of the shoes is How much is the price? 3. Dad saved Xiaoming's education savings for three years (the annual interest rate for three years is 2.7%). After three years, he can withdraw 5405 yuan. How many yuan did he begin to deposit? 4. Divide 100 into two parts so that the result of adding 3 to the first number is equal to that of subtracting 3 from the second number. What are the two numbers? 5. When grandfather and grandson play chess, grandfather wins one game and grandson wins one game and gets three points. After eight games, the two score equally. How many games have they won? 6. A document needs to be printed. It takes Xiao Li 6:00 to do it independently and Xiao Wang 8:00 to do it independently. If they do it together, how long will it take? 7. A harvester harvests a piece of wheat field. In the morning, it harvests 25% of the wheat field, and in the afternoon, it harvests 20% of the remaining wheat field. As a result, there are 6 hectares of wheat field left to be harvested? 8. A shop sells two pieces of clothes at 60 yuan each. One of them earns 20% and the other loses 25%. So does the shop earn or lose, or neither? 9. The speed of train a from place a to place B is 60 km / h, and that of train B from place B to place a at the same time is 90 km / h. It is known that a and B are 200 km apart. How far is the place where the two trains meet from place a? 10. After a and B separated, they went in the opposite direction along the railway track. At this time, a train came to a head-on at an average speed. It took 18 seconds for the train to pass by a, and then it took 17 seconds for the train to pass by B. It is known that the walking speed of both of them is 3.6 km / h. how long is the train? 11. Divide 99 into four numbers, so that the first number of 99 plus 2, the second number minus 2, the third number multiplied by 2, and the fourth number divided by 2, all the results are equal. How should we divide 99 into four numbers? 12. The tombstone of the Greek mathematician dutufan (3rd-4th century) recorded: "one sixth of his life is a happy childhood; after living one twelfth of his life again, his cheeks grow fine whiskers; after living one seventh of his life again, he gets married; after five years, he has a son and feels very happy; but his son only lives half of his whole age; after his son's death, he gets married, He spent four years in agony and died. " (1) What was his age when he got married? (2) What was his age at the time of his death? 13. Xiao Gang and Xiao Ming go to the countryside by bike. They decide to start from home at 8 a.m. in advance. They expect to ride 7.5 kilometers per hour and arrive at their destination at 10 a.m. before they start, they decide to arrive at their destination at 9 a.m., so how many kilometers per hour? 1. If the speed of a ship in still water is 7 km / h and the speed of water is 2 km / h, then the speed of the ship upstream is 0_________ The velocity down the river is__________ . 2. When the car is on a slope, the upward speed is 40 km / h, and the downward speed is 60 km / h, then the average speed of a round trip is 40 km / h___________ . 3. In a section of two track falling Road, two or opposite trains pass by. If train a is 180 meters long and train B is 160 meters long, the wrong time of the two trains is 4 seconds, and it is known that train a is 5 meters faster than train B, then the speeds of train a and train B are_____ 、 1. The distance between a and B is 112 kilometers. A and B drive from a and B at the same time. A travels 4 kilometers more per hour than B. after two hours, they meet. How many kilometers do they travel per hour? 2. A circular track is 400 meters long. A practices cycling, with an average of 550 meters per minute; B practices long-distance running, with an average of 250 meters per minute? 3. Car a and car B drive from south to north. The speed of car a is 48 kilometers per hour, and that of car B is 72 kilometers per hour. After car a leaves for 25 minutes, car B leaves. How many hours will car B catch up with car a? 4. Xiao Wang rowed at a speed of 12 kilometers per hour in still water. Today, he went back and forth to a certain river. It took 10 hours to go upstream and 6 hours to go downstream. The speed of the river was calculated I want it by 11 o'clock. Okay, I'll give you 50 more
• 19. A car runs 130km. At this speed, it runs for 5 hours from place a to place B. how many kilometers are there in a and B? (with proportional solution)
• 20. The distance between the two cities is 564 km. Two trains leave the two cities at the same time and meet each other in 6 hours. It is known that the speed of the first train is 2 km faster than that of the second train per hour. What are the speeds of the two trains?