Question

Cau 1:Tinh A=1/4.9 + 1/9.14 + 1/14.19 + ... + 1/64.69

Cau 2:a.Tim STN nho nhat,biet rang so do khi chia cho 3,cho4,cho 5,cho 6 deu du la 2,con chia cho 7 thi du 3.

b.Tim 2 STN biet tong UCLN va BCNN cua chung = 23

c.Tim STN x;y biet 32x1y chia het cho 45

Answer 1

A=1/(4.9)+1/(9.14)+1/(14.19)+...+1/(64.69)

=1/5.[5/(4.9)+5/(9.14)+5/(14.19)+...+5/(64.69)]

=1/5.(1/4-1/9+1/9-1/14+1/14-1/19+...+1/64-1/69)

=1/5.(1/4-1/69)=1/5.65/276=13/276