2) Tính nhanh

Question

c) 1/2.3 + 1/6.5 + 1/ 10.7+ .... + 1/198.101

Answer 1

A=1/2.3+1/6.5+1/10.7+1/14.9+...+1/198.101A=1/2.3+1/6.5+1/10.7+1/14.9+...+1/198.101

A=1/2(1/1.3+1/3.5+1/5.7+1/7.9+...+1/99.101)A=1/2(1/1.3+1/3.5+1/5.7+1/7.9+...+1/99.101)

4A=2/1.3+2/3.5+2/5.7+2/7.9+...+2/99.1014A=2/1.3+2/3.5+2/5.7+2/7.9+...+2/99.101

4A=1−1/3+1/3−1/5+1/5−1/7+1/7−1/9+...+1/99−1/1014A=1−1/3+1/3−1/5+1/5−1/7+1/7−1/9+...+1/99−1/101

4A=1−/1101=100/1014A=1−1/101=100/101

A=100/101:4=25/101