1, A=1/2 x (1/2 + 1/6 + 1/12+ 1/20 +1/30 +1/42)
A= 1/2 x (1/1x2 + 1/ 2x3 + 1/3x4 + 1/4x5 + 1/5x6 + 1/6x7)
A=1/2 x (1/1 -1/2 +1/2 -1/3 +1/3 - 1/4 + 1/4 -1/5 + 1/5 - 1/6 +1/6 - 1/7 )
A= 1/2 x (1- 1/7)
A= 1/2 x 6/7
A=3/7
2, A= 1/2x(3-1) + 1/3x(4-1) + .....+1/50x(51-1)
A= 1/2x3 - 1/2 + 1/3x4 - 1/3 +.....+ 1/50x51 - 1/50
A= (1/2x3 +1/3x4 +...+1/50.51) - (1/2 +1/3 +....+1/50)
..........................................(giữ nguyên ngoặc thứ 2, làm ngoặc thứ nhất như bài trước)
A=1/2 - 1/51 - (1/2 +1/3 +....+1/50)
A=1/2 - (1/2 +1/3 +....+1/50 + 1/51)
suy ra A<1/2 mà 1/2 <1
nên A<1
