1) 20092009 x 200820082008 - 2009200920009 x 20082008
= 2009 x 10001 x 2008 x 100010001 - 2009 x 100010001 x 2008 x 10001
= 0
2) (x - 7) + x = 5
x - 7 + x = 5
x + x = 5 + 7
2x = 12
x = 12 : 2 = 6
2x - 7 = 5 - x
2x + x = 5 + 7
3x = 12
x = 12 : 3 = 4
3) \(\frac{1}{31}+\frac{1}{32}+...+\frac{1}{90}=\left(\frac{1}{31}+\frac{1}{32}+...+\frac{1}{60}\right)+\left(\frac{1}{61}+\frac{1}{62}+...+\frac{1}{90}\right)\)
30 phân số 30 phân số
\(>\frac{1}{60}.30+\frac{1}{90}.30\)
\(>\frac{1}{2}+\frac{1}{3}=\frac{5}{6}\)
