125125*127-127127*125=15890875-15890875=0.
1/3+1/15+1/35+1/63+1/99+1/143+1/195
=1/3+1/3*5+1/5*7+1/7*9+1/9*11+1/11*13+1/13*15
=2*(2/3+2/3*5+2/5*7+2/7*9+2/9*11+2/11*13+2/13*15)
=2*(2/3+2/3-2/5+2/5-2/7+...+2/13-2/15)
=2*(4/3-2/15)=2*(12/15-2/15)=2*2/3=4/3.
$\frac{1414+1515+1616+1717+1818+1919}{2020+2121+2222+2323+2424+2525}$
=$\frac{14*101+15*101+16*101+17*101+18*101+19*101}{20*101+21*101+22*101+23*101+24*101+25*101}$
=$\frac{14+15+16+17+18+19}{20+21+22+23+24+25}$=$\frac{99}{135}$=$\frac{11}{15}$.
Vậy là xong nhé!!!
Cho mk xin hay nhất nha!!!
Chúc bạn học tốt!!!