\(a,=-17+91-91+17+2011=\left(-17+17\right)+\left(91-91\right)+2011\)

\(=0+0+2011=2011\)

b)\(=\dfrac{4}{7}+\dfrac{1}{6}-0,375.4=\dfrac{31}{42}-\dfrac{3}{2}=-\dfrac{16}{21}\)

\(b)\) \(\dfrac{4}{7}\) + \(\dfrac{5}{6}\) : \(5\) \(- 0,375 .(-2)\)²\(=\) \(\dfrac{4}{7}\) + \(\dfrac{1}{6}\) - \(0,375\) . \(4\)

= \(\dfrac{4}{7}\) + \(\dfrac{1}{6}\) - \(\dfrac{3}{2}\) = \(\dfrac{4.6}{42}\) +  \(\dfrac{1.7}{42}\) - \(\dfrac{3.21}{42}\) = \(-\)\(\dfrac{16}{21}\)

\(\frac{3}{2\times5}+\frac{2}{5\times8}+\frac{3}{8\times11}+...+\frac{3}{602\times605}\)

\(=\frac{5-2}{2\times5}+\frac{8-5}{5\times8}+\frac{11-8}{8\times11}+...+\frac{605-602}{602\times605}\)

\(=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{602}-\frac{1}{605}\)

\(=\frac{1}{2}-\frac{1}{605}=\frac{603}{1210}\)

\(\frac{4}{3\times7}+\frac{5}{7\times12}+\frac{1}{12\times13}+\frac{2}{13\times15}\)

\(=\frac{7-4}{3\times7}+\frac{12-7}{7\times12}+\frac{13-12}{12\times13}+\frac{15-13}{13\times15}\)

\(=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}\)

\(=\frac{1}{3}-\frac{1}{15}=\frac{4}{15}\)

Số số hạng của tổng là

(18-1):1+1=18( số)

Tổng cần tính là

(18+1)x18:2=171

Đáp số: 171

 số các số hạng là:

     (18-1):1+1=18(số)

Tổng là:

   (18+1)x 18 :2 =171

                Đ/S:171

Số số hạng là:

( 18 - 1 ) : 1 + 1 = 18 ( số )

Tổng là: 

( 1 + 18 ) x 18 : 2 = 171

Đáp số : 171

\(12\times68-68\)
\(=68\times\left(12-1\right)\)
\(=68\times11\)
\(=748\)

\(\dfrac{12}{17}>\dfrac{1}{2};\dfrac{5}{36}< \dfrac{1}{2}\\ \Rightarrow\dfrac{12}{17}>\dfrac{5}{36}\)

\(\dfrac{12}{17}\) \(>\) \(\dfrac{5}{36}\)

\(\dfrac{5}{7}\).\(\dfrac{-3}{11}\)-\(\dfrac{5}{7}\).\(\dfrac{8}{11}\)+1\(\dfrac{5}{7}\)

= \(\dfrac{5}{7}\).(\(\dfrac{-3}{11}\) - \(\dfrac{8}{11}\)) + 1\(\dfrac{5}{7}\)

= \(\dfrac{5}{7}\). -1             + 1\(\dfrac{5}{7}\)

= \(\dfrac{-5}{7}\)               +1\(\dfrac{5}{7}\)

= -1

\(\dfrac{5}{7}.-\dfrac{3}{11}-\dfrac{5}{7}.\dfrac{8}{11}+1\dfrac{5}{7}\)

\(=\dfrac{5}{7}.\left(\dfrac{3}{11}-\dfrac{8}{11}\right)+1\dfrac{5}{7}\)

\(=\dfrac{5}{7}.\left(-1\right)+1\dfrac{5}{7}\)

\(=1\)

Ta có:\(\frac{1}{1.5}+\frac{1}{5.9}+\frac{1}{9.13}+......+\frac{1}{81.85}\)

\(=\frac{1}{4}\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+......+\frac{4}{81.85}\right)\)
\(=\frac{1}{4}\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+.......+\frac{1}{81}-\frac{1}{85}\right)\)

\(=\frac{1}{4}.\left(1-\frac{1}{85}\right)\)

\(=\frac{1}{4}.\frac{84}{85}=\frac{21}{85}\)

\(A=\frac{1}{1.5}+\frac{1}{5.9}+...+\frac{1}{81.85}\)

Ta có công thức

\(\frac{a}{b.c}=\frac{a}{c-b}.\left(\frac{1}{b}-\frac{1}{c}\right)\)

\(\Rightarrow A=\frac{1}{4}.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+..+\frac{1}{81}-\frac{1}{85}\right)\)

\(A=\frac{1}{4}.\left(1-\frac{1}{85}\right)\)

\(A=\frac{84}{340}\)

\(\dfrac{45.21.25}{7.15.36}=\dfrac{3.3.3.5.7.5.5}{7.5.3.3.2.3.2}=\dfrac{25}{4}\)

45\7 x 21\15 x 25/36

=45x21x25/7x15x36

=15x3x7x3x25/7x15x3x12

=3x25/12

=75/12

=25/4

-459+181+(-246)+450+(-172) = - 246

ngậm mồm lại đi , tao lơn tuổi hơn mày đấy

Sủa lồn ít thôi :)) lớn tuổi hơn mà ngu vãi cả lồn ạ :))