How to find 3x-21 = x + 19

How to find 3x-21 = x + 19

3x-x=19+21
2x=40
x=20

Finding x.3x + 1 equal to 4 with the properties of equality

3x+1=4
3x + 1-1 = 4-1
3x=3
3x/3=3/3
X = 1 (if both sides of the equation multiply or divide by the same non-zero number, the equation still holds)

Party A and Party B have several books in total, and Party A is known to account for 40% of the total. If Party B gives party a 10 books, the number of books for each person is equal. How many books do Party A and Party B have?

A accounts for 40% of the total,
So B accounts for 60% of the total,
So Party B gives 10% of the total to Party A, then Party A and Party B are equal
So the total is:
10÷10%=100
Number of a:
100 × 40% = 40 copies
Number of Party B:
100-40 = 60
A:
A has 40 books, B has 60 books

Verification: 4A − 3 + a ≥ 7 (a > 3)

∵ a ＞ 3, 4A − 3 + a = 4A − 3 + a − 3 + 3 ≥ 24a − 3 · (a − 3) + 3 = 4 + 3 = 7, if and only if 4A − 3 = a − 3, i.e. a = 5, the equal sign holds

The distance between a and B is 540 km. The two passenger and freight cars run in opposite directions and meet each other in 6 hours. The speed ratio of the two passenger and freight cars is 5:4. How many kilometers does the bus travel per hour?

The sum of the two vehicle speeds is 540 / 6 = 90
Bus speed: 90 * 5 / [5 + 4] = 50 km / h

Application steps of basic inequality

One positive two definite three equal
One positive, that is, the number in the inequality is positive
Determine the basic inequality you want to use according to the direction of inequality
Three phases, etc. to determine whether the condition of taking the equal sign in the basic inequality can be satisfied

The passenger and freight cars run from a and B at the same time. The passenger car runs 66 kilometers per hour, and the freight car runs 44 kilometers per hour. When the freight car runs for eight minutes

The passenger and freight cars run from a and B at the same time. The passenger car runs 66 kilometers per hour and the freight car runs 44 kilometers per hour. When the freight car runs three eighths of the whole journey, the passenger car has exceeded the midpoint of 40 kilometers. How many kilometers are there between a and B
Passenger car speed: freight car speed
=66：44
=3：2
The passenger car passing by accounts for 3 / 8 △ 2 × 3 = 9 / 16
Total length = 40 ÷ (9 / 16-1 / 2) = 640km

As shown in the picture, there is a lamp pole AB in the flowers. Under the light, Xiaoming's shadow length at point D is de = 3 meters, and walking along the BD direction to point G, DG = 5 meters. At this time, Xiaoming's shadow length GH = 5 meters. If Xiaoming's height is 1.7 meters, calculate the height of lamp pole AB (accurate to 0.1 meters)

According to the meaning of the question: ab ⊥ BH, CD ⊥ BH, FG ⊥ BH, (1 point) in RT △ Abe and RT △ CDE, ⊥ ab ⊥ BH, CD ⊥ BH, ⊥ CD ∥ AB, we can prove: △ CDE ⊥ Abe ≁ cdab = Dede + BD ①, (4 points) similarly: fgab = hghg + GD + BD ②, (5 points) and CD = FG = 1.7m

There are 250 workers in team a and team B. if the number of team a is reduced by 1 / 9, the remaining number of team a is 5 more than that of team B. how many people are there in each team?

1-1/9=8/9
B is 5 less than 8 / 9 of A
A: (250 + 5) / (1 + 8 / 9) = 135
B: 250-135 = 115

N natural numbers (1,2,...) are known The total number of each digit of (n) is 1980?

Set n as X bit
The maximum number of digits is (10-1) * 1 + (10 ^ 2-10 ^ 1) 2 +. + (10 ^ X-10 ^ (- 1)) * X
189 when x = 2
3159 when x = 3
So n is estimated to be three digits
（10-1）*1+（10^2-10^1）2+（N-99）*3=1980
So n = 696