Răspuns
Explicație pas cu pas:
[tex]a=\frac{5}{4\cdot 9}+\frac{7}{9\cdot16}+\frac{9}{16\cdot 25}+\frac{11}{25\cdot 36}\\\\a=\frac{9-4}{4\cdot 9}+\frac{16-7}{9\cdot16}+\frac{25-16}{16\cdot 25}+\frac{36-25}{25\cdot 36}\\\\a=\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+\frac{1}{16}-\frac{1}{25}+\frac{1}{25}-\frac{1}{36}\\\\a=\frac{1}{4}-\frac{1}{36}= \frac{9-1}{36}=\frac{8}{36}=\frac{2}{9}\\\\\frac{1}{9}<\frac{2}{9}<\frac{1}{3} |\cdot 9\\\\1<2<3 - adevarat[/tex]
