Question 1: 222 * 39 + 666 * 87 Question 2: 325 * 74 / 74 / 325

Question 1: 222 * 39 + 666 * 87 Question 2: 325 * 74 / 74 / 325

Question 1: 222 * 39 + 666 * 87
=222x3x13+666x87
=666x13+666x87
=666x(13+87)
=666x100
=66600
Question 2: 325 * 74 / 74 / 325
=325x1/325
=1
The title is not very clear. Brackets should be added

1999*1999－（998*1998-（1997*1997-（1996*1996-.（2*2-1）.)

Original formula = 1999 * 1999-1998 * 1998 + 1997 * 1997 -2*2+1
=（1999*1999-1998*1998）+（1997*1997-1996*1996）…… （3*3-2*2）+1
Using the square difference formula, we get: (1999 + 1998) * (1999-1998) + (1997 + 1996) * (1997-1996) +（3+2）*（3-2）+1
Equal to (1999 + 1998) * 1 + (1997 + 1996) * 1 （3+2）*1+1*1
*1 neglect, get 1999 + 1998 + 1997 +3+2+1
(1999 + 1) * 1999 / 2 obtained by arithmetic sequence
It's 1999000
And ha, the landlord has the wrong number. It's 1998,
Note: a * B + A * C = a * (B + C)
Square difference formula: a * A-B * b = (a + b) * (a-b)
Primary school should have learned, but the square difference formula is to expand the content, but it should be said, otherwise it won't come out

I'm stupid
200×19.9-199×19.8
(3 / 8 + 1 / 15) × 8 + 7 / 15

200×19.9-199×19.8
=200*19.9-19.9*198
=19.9(200-198)
=19.9*2
=39.8
(3/8+1/15)*8+7/15
=(3/8)*8+(1/15)*8+7/15
=3+(8/15+7/15)
=3+1
=4
You are not stupid, so am I

12x12x12-12x12x2-10=?
Give me a simple formula! How can I do it!

12x12x12-12x12x2-10
=12x12x(12-2)-10
=12x12x10-10
=10(12x12x-1)
=10x(144-1)
=10x143
=1430