The square of x minus the square of Y

The square of x minus the square of Y

x²-y²=(x+y)(x-y)
If you don't understand, please ask, I wish you a happy study!

In higher mathematics, if the function FX is differentiable in (a, b) and the derivative of FX is greater than 0, then the function FX increases monotonically in (a, b). Why is it an open interval?
Why not a closed interval?

Because derivative is defined as the left derivative equals the right derivative,
If we write "f (x) is differentiable in the closed interval [a, b], then f (a) is called point a non differentiable because there is no left derivative. Similarly, point B is also non differentiable, which contradicts the proposition
So write: "f (x) is derivable in (a, b)"

Simple calculation of 79 1 / 6 △ 19

Changing 79 into 76 plus 3 and 1 / 6 (19 / 6) and then dividing by 19 can be a simple operation to get 4 plus 1 / 6 and 25 / 6

The reciprocal of what plus seven fifths gives a sum of two
It's urgent

The sum of the reciprocal of this number plus seven fifths is two
therefore
The reciprocal of this number is 2 - (7 / 5) = 3 / 5
therefore
It's three fifths, three fifths

12x-9x = 21.6, 0.3x + 0.25x = 1.43, 5x + x-2.5x = 56, how to calculate?

1 question 3x = 21
x=21÷3
x=7
55x = 1.43
x=1.43÷0.55
x=2.6
3 questions 6x-2.5x = 56
3.5x=56
x=56÷3.5
x=16

63 × 37.5% + 17 × 3 / 8 calculated by appropriate method

(63+17)×3/8=80×3/8=30

Can rational numbers be any number?
Can the square of a rational number be any number?

It can't be any number
Rational numbers: rational numbers can be divided into integers and fractions. They can also be divided into positive rational numbers, 0 rational numbers and negative rational numbers

A / (ax-b) + B / (BX + a) = 2 / X (A & sup2; not equal to B & sup2;, AB not equal to 0
How did you get it in the second step?

Judge
The height and area of the trapezoid are doubled
choice.
The upper bottom of a trapezoid is 1.5 times that of the lower bottom, the height is 18 cm, and the area is 180 square cm. The formula for finding the lower bottom is ()
① 180×2÷18÷1.5
② 180×2÷18÷（1+1.5）
③ 540÷18÷（1+1.5）

1. The height and area of the trapezoid are doubled
Area = (upper bottom + lower bottom) * height / 2
two point one

LIM (x - > infinite) n ^ 2 (1-cos (1 / N)) urgent solution,

lim(x→∞)x²(1-cos(1/x))＝lim(x→∞)(1-cos(1/x))／（1／x²）＝lim(x→∞)（－sin(1／x)／x²）／（－2／x³）＝lim(x→∞) sin(1／x)／（ 2／x）＝lim(x→∞) sin(1／x)／（ 1／x）／2＝lim(x...