em mới lớp 6 thui anh ơi 

S=1.2+2.3+3.4+...+99.100

=>3S=3.1.2+3.2.3+3.3.4+...+3.99.100

=(3-0).1.2+(4-1).2.3+(5-2).3.4+...+(101-98).99.100

=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+99.100.101-98.99.100

=99.100.101

=90900

=>S=90900:3=30300

C.cao thửa ruộng hình thang là:
     (110+90,2)/2=100,1(m)
Diện tích thửa ruộng hình thang là:
     (110+90,2)*100,1/2=10020,01(m²)
            Đ.s:

đâu có chia hết

VD n=1

mình trả lời đầu tiên tích mình nha

6. I don't know ........what... to call him , Mister or Doctor

7. we must fing him ,....wherever.. he is

8. If I had today .off........, I would go to the seaside

9. Partice ........makes.. perfect

10. The book I am talking about is that ........one. on the second shelf

1. quiet please ! The headmaster would like to ....give........ a few words

2. you are late ; we're been waiting for you ......since... eight o'clock

3. .......Although... he is wrong , almost everyone admires him

4. I don't think I've lent you this book before , ......have.... I ?

5. many people find ....it...... difficult to give up smoking

(100+1)x(100/2)=5050

Nhầm

...

=> 2x = 18

=> x = 9 (bn sửa lại júp)

...