Question

\(ChoB=\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+...+\dfrac{1}{19}\). Hãy chứng tỏ rằng \(B>1\)

Answer 1

\(B=\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+...+\dfrac{1}{19}\)

\(=\dfrac{1}{4}+\left(\dfrac{1}{5}+\dfrac{1}{6}+...+\dfrac{1}{19}\right)\)

Các phân số \(\dfrac{1}{5}\), \(\dfrac{1}{6}\), \(\dfrac{1}{7}\), ..., \(\dfrac{1}{19}\) đều lớn hơn \(\dfrac{1}{20}\), tất cả có 15 phân số nên:

\(B>\dfrac{1}{4}+\left(\dfrac{1}{20}+\dfrac{1}{20}+...+\dfrac{1}{20}\right)=\dfrac{1}{4}+\dfrac{3}{4}=1\)

Vậy B > 1

Answer 2

e! Chung minh di tai sao lai lam the : phai co ly do chu( ko phai cu thich la ko lam ngay duoc dau