Question

[Tạ Như Ngọc Nga]

tính giá trị biểu thức: 

B = 1.2.3+2.3.4+3.4.5+...+17.18.19

tui dốt toán lém mn giúp tui nha, please :"( toán lớp 7, còn nhìu bài lém huuuu...u ai tốt bụng làm hộ tui đi nạ :(((

sáng chủ nhật nộp rùi :v 

https://scontent.fhan3-1.fna.fbcdn.net/v/t35.0-12/26781720_388359458276208_985270449_o.jpg?oh=ced54383cf06665ce4159d613aaabf11&oe=5A5964B1

https://scontent.fhan3-1.fna.fbcdn.net/v/t35.0-12/26781477_388359544942866_1553842778_o.jpg?oh=94247a3501d233e730cb1a7862035558&oe=5A599834

https://scontent.fhan3-1.fna.fbcdn.net/v/t34.0-12/26754849_388359671609520_1056762744_n.jpg?oh=39fac53855f8a53001a0122d6533168c&oe=5A594DD9 

Answer 1

[lê quang tuyến]

B = 1.2.3+2.3.4+3.4.5+....+17.18.19

4B=1.2.3.4+2.3.4.4+3.4.5.4+....+17.18.19.4

4B=1.2.3.4+2.3.4.(5-1)+3.4.5.(6-2)+.....+17.18.19.(20-16)

4B=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+3.4.5.6+.....-16.17.18.19+17.18.19.20

4B=17.18.19.20

B=116280:4=29070

Answer 2

[Nguyễn Anh Quân]

4B = 1.2.3.4+2.4.4.4+.....+17.18.19.4

     = 1.2.3.4+2.3.4.(5-1)+.....+17.18.19.(20-16)

     = 1.2.3.4+2.3.4.5-1.2.3.4+....+17.18.19.20-16.17.18.19

     = 17.18.19.20

=> B = 17.18.19.20/4 = 29070

Tk mk nha