đề phải là:

1*2+2*3+...+99*100

Gọi biểu thức trên là A, ta có :

A = 1x2 + 2x3 + 3x4 + 4x5 + ...+ 99x100

A x 3 = 1x2x3 + 2x3x3 + 3x4x3 + 4x5x3 + ... + 99x100x3

A x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + 4x5x(6-3) + ... + 99x100x(101-98)

A x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + 4x5x6 - 3x4x5 + ... + 99x100x101 - 98x99x100.

A x 3 = 99x100x101

A = 99x100x101 : 3

A = 333300

ghê vậy

ban dung cai chan quang cao di la ok ma

Let's.............go to the sea for a change...............................................................?

Bai 2 :
1.speaking
2.famous
3.entertainment
4. second
5.sunny
6.better
7.longest
8.nationality
9.electricity
10.himself

=>It is easy to find a book either an author or a title.
Those / book / black / library /be / vietnamese

=>Those book at the back of the library are in Vietnamese.
Your father / watch /  TV / sitting room / the moment ?

=>Is your father watching TV in the sitting room at the moment?
Magazines / newspapers / be / the rack / conner / the left 

=>Magazines and newspaper are on the rack at the corner on the left.
Our library / one / the / large / libreries / city 

=>Our library is one of the largest libraries in our city.

9 * 9 : 9 + ( 9 : 9 )

I like drinking orange juice better than lemonade.

⇒⇒ I prefer ____drinking orange juice to drinking lemonade ones_________________________.

M: tell me about your (1).....class.....,Lan. What time does your class start?

L:my class always starts at 7.30.

M:(2)...When .......do you have physics?

L:I have physics(3)..class.......on Monday and Friday

M:(4).....When .......do they start?

L:My(5)....(thua).......Physics class is on Monday at 8.30. On Friday,I have Physics at 9.50

M:What(6) ..(thua)......classes do you have on Friday

L:I have English, History(7)......and....Physical Education

M:What's your(8)....favorite.......subject,Lan

L:I like literature(9)...because.....It is an interesting and helpful subject. And do you like literature,Mai

M No.My  favourite subject is Math . It's difficult, (10)...but...fun.