A= 1x2+2x3+3x4+...+98x99
A x 3= 1x2 x (3-0) +2x3x (4-1)+3x4 x (5-2)+...+98x99x (100-97)
= 1x2x3+2x3x4+......98x99x100- (1x2x0+ 2x3x1+....+ 98x99x97)
= 98x99x100.
Tính tổng: 1x2+2x3+3x4+...+98x99
Tính A= 1/1x2+1/2x3+1/3x4+........+1/98x99+1/99x100
tính nhanh:5/1x2 + 5/2x3 + 5/3x4 ...+ 5/98x99 + 5/ 99x100
S= 1x2+2x3+3x4+4x5+...+98x99+99x100
3/1x2-5/2x3+7/3x4-9/4x5+.......-197/98x99
1/1x2 + 1/2x3 + 1/3x4 +......+1/98x99 + 1/99x100
S=1x2+2x3+3x4+4x5+...+98x99
Tìm S:))
1x2+2x3+3x4+4x5+5x6+...+98x99
Tính nhanh
Giải đầyđủ nha mình cần gấp lắm hôm nay phải có
Tính
A=1x2x3+2x3x3+3x4x3+4x5x3+....+98x99x3
B=1x2+2x3+3x4+4x5+...+98x99
C=1x1+2x2+3x3+4x4+5x5+...+98x98
