Question

[kim thị mai trang]

Cho biết 1x20+2x19+3x18+...+20x1= 1540

Tính A=1+2+4+7+11+16+.....+211

Answer 1

[Phan Hải Đăng]

ta có 

a=1+2+4+...........+211

=1+[1+1]+[1+1+2]+[1+1+2+3]+..................+[1+1+2+3+...........+20] =[1+......+1]+[1+1+.....+1]+[2+......+2]+..........+[19+19]+20

=21+[1*20+2*19+...............+19*2+20]

   =1561