This is the third grade math problem. Who can solve it! Two children and an adult came to the same bank of the river. On the bank, they saw a small boat that could carry two children or an adult. All three people had to cross the river in this boat. What should they do? If it took three minutes to cross the river at a time, how long would it take at least to cross the river?

Step one: two children cross the river
Step 2: one of the children rowed back
Step 3: Adults row the boat
Step 4: the child who has rowed will row the boat back
Step 5: two children row together
Five minutes in each step, 15 minutes in total
Choose me!

I want to ask: under what circumstances or restrictions, the triangle rule is used, and under what circumstances or restrictions, the parallelogram rule is used?
Want to know when to use, always understand very vague
But also want to know the use of both restrictions
It's better to explain if there are any skills to use~
Thank you very much. I'm in a hurry!
From the mathematical vector or physical aspects can be, preferably the composition of physical forces or decomposition!
Thank you very much~~

In fact, the two rules are essentially the same, there is no difference, just the starting point of the vector is different, the end point of a vector is the starting point of another vector, use the triangle, the same starting point of two vectors use the parallelogram rule, but you can also translate a vector and then use the triangle rule, in fact, after all

If we know that a triangle has three sides, a, B and C satisfy the square of the relation (a + b) + (a-b) C = 0, then the triangle must be -?

It should be the square of (a-b) + (a-b) C = 0
（a-b）(a-b+c)=0
The sum of the two sides is greater than 0
That is, a + C-B > 0
therefore
a-b=0
a=b
therefore
A triangle is an isosceles triangle

Several kinds of monotonicity problems of a function in senior one mathematics

Generally, if the B power of a (a is greater than 0, and a is not equal to 1) is equal to N, then the number B is called the logarithm of n with a as the base, which is recorded as log an = B, where a is called the base of logarithm, and n is called the true number

Mathematics of senior two -- solving problems with derivative
Let f (x) = x & sup3; (cube of x) - (1 / 2) x & sup2; - 2x + 5

f'=3x^2-x-2=(3x+2)(x-1)
F '= 0 root is X1 = - 2 / 3, X2 = 1
The maximum value f (- 2 / 3) = 157 / 27 and the minimum value f (1) = 7 / 2, so the maximum value of F in [- 1,2] is f (- 2 / 3) = 157 / 27
Because when x belongs to [- 1,2], f (x) Fmax = f (- 2 / 3) = 157 / 27

Vector A / / vector B x1y2-x2y1 = 0 why?

According to the fact that the two vectors are parallel, their K values are the same
That is, X1 / Y1 = x2 / Y2
It can be concluded that x1y2-x2y1 = 0

If x and y are known to satisfy x square - XY + y square = 1, then what is the sum of the maximum and minimum of x square + y square? It is better to have a process

-1

Oral arithmetic exercises. Off form exercises. Equation solving exercises
200 channels each

The solution of a system of equations {X-Y = 2A, x + 3Y = 1-5a} about X and Y satisfies x + Y > - 1, and the value range of a is obtained

A is less than 1

Which is the formula for multiplying the largest three digits by the largest two digits composed of the five numbers 1, 2, 3, 4, 5? (the product is the largest)

52*431=22412