Calculation of 800-2 × 800 × 799 + 799 by factorization

Calculation of 800-2 × 800 × 799 + 799 by factorization

800-2 × 800 × 799 + 799. = (800-799) = 1. I think you should understand: (a-b) = a-2ab + B. here we use this

Translate the square ABCD along the AC direction to the square A / B / C / D /, and the area of their overlapping part is 1 / 4 of the area of the square ABCD, if AC = 2cm,
Find the distance aa'of square moving
Picture address
Use ∵
Write in the form of

For example, ABCD and a~
The overlap area is 1 / 4 of the square ABCD area
A ~ is at the midpoint of the AC line
∵AC=2cm,
∴AA~=1cm

Square of X + 6x + 9 / square of X-9

=(x + 3) square / [(x + 3) (x-3)]
=(x+3)/(x-3)

Let a be an n * m type matrix, B be an M * n type matrix, and I be an n-order unit matrix. If AB = I, it is proved that the column vectors of B are linearly independent

Because n = R (in) = R (AB)

Known: as shown in the figure, in the triangle ABC, ∠ ABC = 45 °, CD vertical AB, be vertical AC, CD and be intersect with point F, prove: BF = AC

Proof: because CD is perpendicular to ab
So angle BDC = angle ADC = 90 degrees
Because angle BDC + angle ABC + angle DCB = 180 degrees
Angle ABC = 45 degrees
So the angle DCB is 45 degrees
So angle ABC = angle DCB = 45 degrees
So BD = CD
Because angle ADC + angle ACD + angle a = 180 degrees
So angle a + angle ACD = 90 degrees
Because be vertical AC
So the angle AEB is 90 degrees
Because angle AEB + angle a + angle ABF = 180 degrees
So angle a + angle ABF = 90 degrees
So angle ABF = angle ACD
So triangle BDF and triangle CDA are congruent (ASA)
So BF = AC

Given function f (x) = a (x-1 / x) - 2lnx (a ∈ R)
Let g (x) = - A / x, if there is at least one x0 ∈ [1, e], such that f (XO) > G (XO) holds, find the value range of real number a

Finding line surface angle by space vector
If you want to prove that the line and plane are perpendicular, there are two ways to use the space vector. One is to find the plane normal vector, and let the normal vector and the line vector dot multiply so that x1y2 = x2y1. Two intersecting vectors in the plane multiplied by the line vector dot are equal to 0. Are these two ways OK?

Yes, but the second one uses more. After all, it's troublesome to find the normal vector. Moreover, the first one is not only x1y2 = x2y1,
Because it's a space vector, there are three-dimensional coordinates,
It should be x1y2 = x2y1, and x1z2 = x2z1 (this is a necessary and sufficient condition for two 3D vectors to be parallel)

As shown in the figure, in △ ABC, ab = AC, CD is the height on the edge of AB, and the proof is: ∠ BCD = 12 ∠ a

It is proved that a is AE ⊥ BC in E, Cd in F, and ∫ BAE + ⊥ B = 90 ° and ab = AC, ∫ BAE = 12 ⊥ BAC. And ∫ CD ⊥ AB, ∫ BCD + ⊥ B = 90 ° and ∫ BAE = ∫ BCD. ∫ BCD = 12 ∫ a

The value of the unknowns 4x = 2x

4X = 2x minus 2x on both sides
4x-2x=0
2x=0
x=0
The value of the unknown x is 0

Given that the coordinates of points a and B are (1,2) and (4,2) respectively, the vector coordinates obtained by translating vector AB according to vector a = (− 1,3) are ()
A. （3，0）B. （3，5）C. （-4，3）D. （2，3）

If the coordinates of points a and B are (1,2) and (4,2) respectively, then the vector AB = (3,0). After the vector AB is translated according to the vector a = (− 1,3), the resulting vector is: (3,0). (its length and direction remain unchanged.) so a is selected