b:\(1+2+3+...+n=\dfrac{n\left(n+1\right)}{2}\)

c: \(1+3+5+...+37\)

Số số hạng là:

(37-1):2+1=19(số)

Tổng là: \(\dfrac{38\cdot19}{2}=19\cdot19=361\)

\(\frac{1}{7}\cdot\frac{2}{3}+\frac{5}{6}:\frac{3}{10}-\frac{2}{3}\cdot\frac{3}{7}\)

\(=\left(\frac{1}{7}\cdot\frac{2}{3}-\frac{2}{3}\cdot\frac{3}{7}\right)+\frac{5}{6}\cdot\frac{10}{3}\)

\(=\frac{2}{3}\cdot\left(-\frac{2}{7}\right)+\frac{5}{3}\cdot\frac{5}{3}\)

\(=-\frac{4}{21}+\frac{25}{6}=\frac{167}{42}\)

Xin lỗi không nhanh được 

Bạn ơi câu này trả lời r mà

3.12:0.25+12.48*3-3.12*6

=3.12*4+3.12*12-3.12*6

=3.12*(4+12-6)

=3.12*10

=31.2

= 3,12 x 4 + 3,12 x 4 x 3 - 3,12 x 6

= 3,12 x 4 +3,12 x 12 - 3,12 x 6

= 3,12 x ( 4 + 12 - 6 )

= 3,12 x 10

= 31,2

    3,12 : 0,25 + 12,48 x 3 - 3,12 x 6

=  3,12 x 4 + 3,12 x ( 4 x 3 ) - 3,12 x 6

= 3,12  x 4 + 3,12 x 12 - 3,12 x 6

= 3,12 x ( 4 + 12 - 6 )

= 3,12 x         10

= 31,2

3,12 : 0,25 + 12,48 x 3 - 3,12 x 6

=3,12 : 1/4 + 3,12 x 4 x 3 - 3,12 x 6

=3,12 x 4 + 3,12 x 12 - 3,12 x 6

=3,12 x (4 + 12 - 6)

=3,12 x 10

=31,2

Tính bằng các thuận tiện nhất: 10/7 x 2/3 + 6/5 : 3/10 - 2/3 x 3/7

trả lời:

10/7 x 2/3 + 6/5 : 3/10 - 2/3 x 3/7=10/7 x 2/3 + 6/5. 10/3 - 2/3 x 3/7=10/7 x 2/3 + 2/3.2- 2/3 x 3/7

=2/3.(10/7+2-3/7)=2/3.3=2

   10/7 x 2/3 + 6/5 : 3/10 - 2/3 x 3/7
= 10/7 x 2/3 + 6/5 x 10/3 - 2/3 x 3/7
= 10/7 x 2/3 + 6/5 x 5 x 2/3 - 2/3 x 3/7
= 10/7 x 2/3 + 6 x 2/3 - 2/3 x 3/7
= 2/3 x (6 + 10/7 - 3/7)
= 2/3 x (6 + 1)
= 2/3 x 7
= 14/3
CHúc bạn học tốt
 

\(\frac{10}{7}\cdot\frac{2}{3}+\frac{5}{6}:\frac{3}{10}-\frac{2}{3}\cdot\frac{3}{7}\)

\(=\left(\frac{10}{7}\cdot\frac{2}{3}-\frac{2}{3}\cdot\frac{3}{7}\right)+\frac{5}{6}\cdot\frac{10}{3}\)

\(=\frac{2}{3}\left(\frac{10}{7}-\frac{3}{7}\right)+\frac{5\cdot10}{6\cdot3}\)

\(=\frac{2}{3}\cdot1+\frac{25}{9}\)

\(=\frac{2}{3}+\frac{25}{9}\)

\(=\frac{31}{9}\)

4 6 .   9 5 +   6 9 .   120 8 4 .   3 12 − 6 11 = 2 2 6 . 3 2 5 + 2 9 .3 9 .2 3 .3.5 2 3 4 .3 12 − 2 11 .3 11 = 2 12 .3 10 + 2 12 .3 10 .5 2 12 .3 12 − 2 11 .3 11 = 2 12 .3 10 1 + 5 2 11 .3 11 2.3 − 1 = 2.6 3.5 = 4 5

\(28+35+62+95+73+69+27+1\)

\(=\left(28+62\right)+\left(35+95\right)+\left(73+27\right)+\left(69+1\right)\)

\(=90+130+100+70=390\)

(28+62)+(35+95)+(73+27)+(69+1)=

90+130+100+70=390

Đs:390