Question

A=10+13+.......+79+81

B=1/1.2+1/2.3+1/3.4+.......1/12.20

C=3/7x4/13+3/7+5 4/7

Answer 1

Phần A sai nha phải là: 10 + 13 + ... + 79 + 82

đáp án:

SSH: A = ( 82 - 10 ) : 3 + 1 = 25

Tổng: ( 10 + 82 ) . 25 : 2 = 1150

Answer 2

a) A = 10+13 + ...+79 + 81

A = ( 79+10) x 24 : 2 + 81

A = 89 x 24 : 2 +81

A = 1068+ 81

A= 1149

b) chỗ " 1/12.20" phải là 1/12.13 chứ !

 \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{12.13}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{12}-\frac{1}{13}\)

\(=1-\frac{1}{13}\)

\(=\frac{12}{13}\)

c) \(C=\frac{3}{7}\times\frac{4}{13}+\frac{3}{7}+5\frac{4}{7}\)

\(C=\frac{3}{7}\times\left(\frac{4}{13}+1\right)+\frac{39}{7}\)

\(C=\frac{3}{7}\times\frac{17}{13}+\frac{39}{7}\)

\(C=\frac{51}{91}+\frac{39}{7}\)

\(C=\frac{558}{91}\)