Đáp án:
a, A=1/5.6 + 1/6.7 +...+ 1/24.25
A=1/5-1/6+1/6-1/7+...+1/24-1/25
A=1/5-1/25
A = 4/25
b, B= 5^2/1.6 + 5^2/6.11 +...+ 5^2/26.31
5B=5/1.6 + 5/6.11 +...+5/26.31
5B=1-1/6+1/6-1/11+...+1/26-1/31
5B=1-1/31
5B=30/31
B=6/31
c, C=1/7+1/91+1/247+1/475+1/755+1/1147
C=1/1.7 + 1/7.13 + 1/13.19 + 1/19.25 + 1/25.31 + 1/31.37
6C=6/1.7 + 6/7.13 +...+6/25.31 + 6/31.37
6C=1-1/7+1/7-1/13+...+1/25-1/31+1/31-1/37
6C=1-1/37
6C=36/37
C=6/37
d, D=10/7.12 + 10/12.17 + 10/17.22 +...+10/502.507
D/2=5/7.12 + 5/12.17 + 5/17.22 +....+ 5/502.507
D/2= 1/7 -1/12+1/12-1/17+1/17-1/22+...+1/502-1/507
D/2=1/7-1/507
D=(1/7-1/507)/2
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