Solving equation x × x + 3x = 1

Solving equation x × x + 3x = 1

It's x ^ 2 + 3x = 1
x^2+3x=1 ,
x^2+3x-1=0 ,
a =1 ,b = 3 ,c = -1 ,
△=b^2-4ac=9+4=13 ,
x1=(-3-√△)/2=(-3-√13)/2 ,x2=(-3+√△)/2=(-3+√13)/2 .

Factorization X & # 178; - 3x + 1,
For example, I know the root formula, and I know the result, but I don't understand why I use (x-x1) (x-x2) in the end. Please explain it in detail

This is a formula, you can apply it directly, no matter
x²-3x+1=(x-2)(x-1)

1996×1999419941994－1994×199619961996＝?199999＋19999＋1999＋199＋19＝?

1996 × 1999419941994-1994 × 199619961996
1996×199419941994－1994×199619961996
=1996×199419940000+1996×1994-199619960000×1994-1996×1994
=10000×(1996×19940000+1996×1994-19960000×1994-1996×1994)
=10000×0
=0
199999＋19999＋1999＋199＋19
=200000-1+20000-1+2000-1+200-1+20-1
=222220-5
=222215

1. How to read 19, 199, 1999, 19999, 199999 in English?
I forgot how to read irregular numbers in English
For example, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22
20、30、40、50、60、70]
And how to read 1986?

1 one
19 nineteen
199 one hundred and ninety-nine
1999 one thousand nine hundred and ninety-nine
19999 nineteen thousand nine hundred and ninety-nine
199999 one hundred and ninety-nine thousand nine hundred and ninety-nine
Look up the rest in the dictionary. I'm dead www.dict.cn
1986 nineteen eighty-six