Question

[Lê Tâm Thư]

tính tổng : A=1.5 + 5.9 + ....+97.101+101.105

B=1.2^2+2.3^2+3.4^2+....+99.100^2

C=1.2+3.4+5.6+7.8+...+99.100

D=1.2.3+2.3.4+...+98.99.100

Answer 1

[Nguyễn Anh Quân]

Mình làm mẫu 1 bài nha !

Có : 12A = 1.5.12+5.9.12+....+101.105.12

= 1.5.12+5.9.(13-1)+.....+101.105.(109-97)

= 1.5.12+5.9.13-1.5.9+.....+101.105.109-97.101.105

= 1.5.12-1.5.9+101.105.109

= 1155960

=> A = 1155960 : 12 = 96330

Tk mk nha

Answer 2

[Nguyễn Anh Quân]

Có : 4D = 1.2.3.4+2.3.4.4+....+98.99.100.4

= 1.2.3.4+2.3.4.(5-1)+.....+98.99.100.(101-97)

= 1.2.3.4+2.3.4.5-1.2.3.4+......+98.99.100.101-97.98.99.100

= 98.99.100.101

=> D = 98.99.100.101/4 = 24497550