Question

Tính bằng cách thuận tiện

a) \(\frac{3}{5}+\frac{4}{7}+\frac{2}{5}+\frac{1}{7}+\frac{2}{7}\)                               b)  \(\frac{4}{9}+\frac{8}{9}+\frac{12}{9}+\frac{16}{9}+........+\frac{48}{9}+\frac{52}{9}+\frac{56}{9}\)

giúp mình với ai nhanh mình thưởng cho 2 tick

Answer 1

a) \(\frac{3}{5}+\frac{4}{7}+\frac{2}{5}+\frac{1}{7}+\frac{2}{7}\)

\(=\left(\frac{3}{5}+\frac{2}{5}\right)+\left(\frac{4}{7}+\frac{1}{7}+\frac{2}{7}\right)\)

\(=\frac{5}{5}+\frac{7}{7}=1+1=2\)

Answer 2

a) \(\frac{3}{5}+\frac{4}{7}+\frac{2}{5}+\frac{1}{7}+\frac{2}{7}\)

\(=\left(\frac{3}{5}+\frac{2}{5}\right)+\left(\frac{4}{7}+\frac{1}{7}+\frac{2}{7}\right)\)

= 1 + 1

= 2

b) \(\frac{4}{9}+\frac{8}{9}+\frac{12}{9}+\frac{16}{9}+...+\frac{48}{9}+\frac{52}{9}+\frac{56}{9}\)

\(=\frac{4+8+12+16+...+48+52+56}{9}\)

Xét 4 + 8 + 12 + 16 + ... + 48 + 52 + 56

Số các số hạng là: 

      (56 - 4) : 4 + 1 = 14 (số)

4 + 8 + 12 + 16 + ... + 48 + 52 + 56 = (56 + 4) x 14 : 2 = 420

\(\frac{4+8+12+16+...+48+52+56}{9}=\frac{420}{9}=\frac{140}{3}\)

Answer 3

a)=(3/5 + 2/5)+(4/7 + 1/7 + 2/7)=1+1=2

b)=sshl là:(56/9 - 4/9) : 2 +1=35/9

tổng là:(56/9 + 4/9)* 35/9 :2=350/27

Answer 4

a)

3/5 + 4/7 + 2/5+ 1/7+2/7

=(3/5 +2/5) + (4/7 +1/7 + 2/7)

=5/5 +7/7

=1+1

=2

b) 4/9 + 8/9 + ... +56/9

= 4(1/9 +2/9 +3/9+...+14/9)

=4(\(\frac{\text{1+2+3+4+...+14}}{9}\))

=4(\(\frac{\frac{14\left(14+1\right)}{2}}{9}\))

=4*\(\frac{105}{9}\)

=140/3