Balmer formula, the first number is nine fifths, find the seventh number

Balmer formula, the first number is nine fifths, find the seventh number

(n+2)²/（n+2）²-4
The numerator is the square of (term + 2) (for example, the first is 9 / 5, and the term is 1). The denominator is 7, 9, 11, 13 added to the previous denominator （7+2=9,9+2=11……）
The result is 81 / 77

The quotient of 1 / 2 divided by 2 / 5 of an addend. If it's 9 / 4, find the number. (use the comprehensive formula to solve)

2/5×9/4÷1/2
=9/5

It is known that a times 4 / 3 is equal to 11 / 12 times B is equal to 15 / 15 times C, and a, B, C are not equal to 0. Try to arrange a, B, C in the order from large to small, and explain the reason

b>c>a
15 / 15 times C can be regarded as C. let C = 1
If 4 / 3 is greater than 1, make a multiply 4 / 3 equal to 1, then a is less than 1
If 11 / 12 is less than 1, B is greater than 1
So b > C > a

It is known that the domain of definition of the function y = AX2 + 2aX + A is r, and the inequality x2-x-a2 + a about X is less than 0

When a = 0, the function y = 0 satisfies the domain of definition R, then the inequality x2-x-a2 + a < 0 about X is equivalent to x2-x < 0, and the solution is 0 < x < 1. The solution set of the inequality is (0, 1). When a ≠ 0, if the domain of definition of the function y = AX2 + 2aX + A is r, because y = a (x + 1) 2, we can get a > 0

Sixth grade mathematics
A bundle of wires is wound in 8 circles with a radius of 0.2m. How many meters is this bundle of wires?
A round flower bed is 15.7 meters in circumference and 3 meters in width. How many meters in circumference?

8*2*π*0.2=10.048m
2*π*（15.7/2/π+3）=34.54m
The answer is wrong

For the rational number XY, a new operation is defined: X * y = ax + by + C, where ABC is a constant, and the right side of the equation is the general operation of addition and multiplication
Given that 1 * 2 = 9 (- 3) * 3 = 60 * 1 = 2, find the value of 2 * (- 7)

{9=1*2=a+2b+c
{6=(-3)*3=-3a+3b+c
{2=0*1=b+c
By solving this system of linear equations with three variables, we get a = 2, B = 5, C = - 3
Then we use X * y = 2x + 5y-3
2*(-7)=2*2-5*7-3=-34

30 minus 3 equals 27 divided by 30 = 90 percent of pass

No
We have to multiply it by 100% to get the qualified rate
(30-3) / 30 times 100% = 90%
This is the pass rate

If positive integers x and y satisfy 2x + Y -- xy = 0, find the minimum value of X + y

First, convert the equation to y = 2x / (x-1)
(x + y) derivative x, get 1 + (2x / (x-1)) 'make it equal to 0, judge how much X has the maximum value, get x = 0 or x = 2, remove x = 0, because x is a positive integer, so x = 2
In this case, y = 4, x + y = 6

Shampoo 15 yuan per bottle, shopping malls to carry out promotional activities, buy 4 bottles get 1 free. Buy 4 bottles at a time, how much cheaper per bottle?

15-15 × 4 ^ (4 + 1) = 15-15 × 4 ^ 5 = 15-60 ^ 5 = 15-12 = 3 (yuan) a: it's 3 yuan cheaper per bottle

X is greater than 0, y is greater than 0, and 2 / x + 8 / y = 1

1≥2√(2/x*8/y)=8√(1/xy)
√(xy)≥8
xy≥64
Minimum = 64
No maximum