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chứng tỏ :
1/1*2 + 1/3*4+.......+1/49*50 = 1/26 +1/27 +.......+1/50
1/26+1/27+1/28+...+1/49+1/50=1-1/2+1/3-1... <=>2/26+2/28+2/30+...+2/50=1-1/2+1/3-1... <=>1/13+1/14+1/15+...+1/25=1-1/2+1/3-1... <=>2/14+2/16+2/18+...2/24=1-1/2+1/3-1/... <=>1/7+1/8+1/9+...+1/12=1-1/2+1/3-1/4+... <=>2/8+2/10+2/12=1-1/2+1/3-1/4+1/5-1/6 <=>1/4+1/5+1/6=1-1/2+1/3-1/4+1/5-1/6 <=>2/4+2/6=1-1/2+1/3 <=>1/2+1/3=1-1/2+1/3 <=>2/2=1
2/2=1
1/26+1/27+1/28+...+1/49+1/50=1-1/2+1/3-1...
<=>2/26+2/28+2/30+...+2/50=1-1/2+1/3-1...
<=>1/13+1/14+1/15+...+1/25=1-1/2+1/3-1...
<=>2/14+2/16+2/18+...2/24=1-1/2+1/3-1/...
<=>1/7+1/8+1/9+...+1/12=1-1/2+1/3-1/4+...
<=>2/8+2/10+2/12=1-1/2+1/3-1/4+1/5-1/6
<=>1/4+1/5+1/6=1-1/2+1/3-1/4+1/5-1/6
<=>2/4+2/6=1-1/2+1/3
<=>1/2+1/3=1-1/2+1/3
<=>2/2=1
2/2=1