According to the meteorological observation data of a certain place, the temperature will decrease by about 6 degrees for every 1km increase in altitude. If the surface temperature of the area is 21 degrees, the temperature of a place in the upper air is - 39 degrees Find the height above the ground

According to the meteorological observation data of a certain place, the temperature will decrease by about 6 degrees for every 1km increase in altitude. If the surface temperature of the area is 21 degrees, the temperature of a place in the upper air is - 39 degrees Find the height above the ground

【21-（-39）】/6=10km.
The height above the ground is 10km (vertical, in the troposphere)

When the altitude increases by 1 km, the temperature decreases by 6 ° C. if the ground temperature is 21 ° C, the upper air temperature is - 39 ° C, and the altitude is? Km

21-6X=-39
x=10
The answer is 10 kilometers

The distance between city a and city B is 450 km. Two cars drive from the two cities at the same time and meet three hours later. It is known that the speed ratio of car a and car B is 3:2, and how many kilometers each car travels per hour
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Speed and speed: 450 △ 3 = 150 km / h
A speed: 150 △ 3 + 2 × 3 = 90 km / h
B speed: 150-90 = 60 km / h

A two place decimal point moves one place to the right, and the subtraction between the decimal point and the original number is 651.78. What is the original number?

Let the original number be X. after the decimal point is moved one place to the right, the number becomes 10x,
10X-X=651.78
X = 72.42, that is, the original number is 72.42

A passenger car and a train leave from a and B at the same time. The passenger car runs 80 kilometers per hour, and the freight car runs 60 kilometers per hour
How many kilometers long is the railway between a and B when the two cars meet

(60+80)x5=140x5=700km
The railway is 700 km long

Let X & sup2; - 4x-2 (k-1} = 0 have two real roots x 1, x 2. Is there any case where x 1 + x 2 is greater than x 1 · x 2

From X & # 178; - 4x-2 (k-1) = 0,
By Weida's theorem: X1 + x2 = 4,
x1·x2=-2（k-1）
x1-x2-x1·x2
=4-（-2（k-1)
=4+2k-2＞0,
2k＞-2,k＞-1,
When k ＞ - 1, x 1 + x 2 is larger than x 1 · x 2

The two trains a and B run from 580 kilometers away and meet after 5 hours. It is known that the speed ratio of the two trains is 14:15
How many kilometers do the two trains travel per hour?

The sum of the speed of train a and B is 580 / 5 = 116 km / h
Speed of train a = 116 * 14 / (14 + 15) = 1624 / 29 = 56 km / h
B train speed = 116-56 = 60 km / h

The average of the three numbers is 25, and the ratio of the three numbers is 7:5:3
The smallest of the three numbers is ()

Let the three numbers be 7x, 5x and 3x respectively
Then (7x + 5x + 3x) / 3 = 25
The solution is x = 5
So the minimum number is 3x = 3 * 5 = 15

The distance between a and B is equal to the distance between B and C. the speed of B is 80% of the speed of A. It is known that B starts 11 minutes earlier than a, but stops at B for 7 minutes, and a keeps on driving to C. finally, B arrives at C 4 minutes later than a. then, after B starts, B starts______ The first car will overtake the second car in 15 minutes

Try to find out such a four digit number. The square of the sum of the two digits formed by the first two digits and the last two digits is exactly equal to the four digit number

Let the two double digits be x, y, ■ (x + y) 2 = 100x + y.x2 + 2 (y-50) x + (y2-y) = 0.b2-4ac = 4 (y-50) 2-4 (y2-y) = 4 (2500-99y) ≥ 0, and the solution is y ≤ 252599. When y ≤ 252599, the original equation has a solution