Bài 14:
a: \(A=\frac{1985\cdot1987-1}{1980+1985\cdot1986}\)
\(=\frac{1985\cdot1986+1985-1}{1985\cdot1986+1980}=\frac{1985\cdot1986+1984}{1985\cdot1986+1980}>1\)
b: \(A=\frac{5\left(11\cdot13-22\cdot26\right)}{22\cdot26-44\cdot54}\)
\(=\frac{5\cdot11\cdot13\cdot\left(1-2\cdot2\right)}{22\cdot\left(26-2\cdot54\right)}=\frac{5\cdot13}{2}\cdot\frac{1-4}{26-108}\)
\(=\frac{65}{2}\cdot\frac{-3}{-82}=\frac{65}{2}\cdot\frac{3}{82}=\frac{195}{164}=1+\frac{31}{164}\)
\(B=\frac{138^2-690}{137^2-548}=\frac{138\left(138-5\right)}{137\left(137-4\right)}=\frac{138}{137}\cdot\frac{133}{133}=\frac{138}{137}\)
\(=1+\frac{1}{137}=1+\frac{31}{4247}\)
mà 31/164>31/4247(164<4247)
nên A>B
