Question

[Cô bé chăm học]

Tính nhanh

a, F= 15/90.94 + 15/94.98 + 15/89.102+........+15/146.150

b, G= 10/56 + 10/140 + 10/260 + ........ + 10/1400

Answer 1

[Đinh Đức Hùng]

\(F=\frac{15}{90.94}+\frac{15}{94.98}+\frac{15}{98.102}+....+\frac{15}{146.150}\)

\(\Rightarrow F=\frac{15}{4}\left(\frac{15}{90}-\frac{15}{94}+\frac{15}{94}-\frac{15}{98}+....+\frac{15}{146}-\frac{15}{150}\right)\)

\(\Rightarrow F=\frac{15}{4}\left(\frac{1}{6}-\frac{1}{10}\right)=\frac{15}{4}\left(\frac{5}{30}-\frac{3}{30}\right)=\frac{15}{4}.\frac{1}{15}=\frac{15}{60}=\frac{1}{4}\)