a) \(31\cdot72+\left(-31\right)\cdot70-2\cdot70\)

\(=31\cdot72-31\cdot70-2\cdot70\)

\(=31\cdot\left(72-70\right)-2\cdot70\)

\(=31\cdot2-2\cdot70\)

\(=2\cdot\left(31-70\right)\)

\(=2\cdot-39\)

\(=-78\) 

b) \(25\cdot\left(32+17\right)-32\cdot\left(25+47\right)\)

\(=25\cdot32+25\cdot17-32\cdot25-32\cdot47\) 

\(=25\cdot\left(32-32+17\right)-32\cdot47\)

\(=25\cdot17-32\cdot47\)

\(=425-1504\)

\(=-1079\)

4: \(=\dfrac{2+3}{7}+\dfrac{1+6}{9}-\dfrac{5}{6}=\dfrac{5}{7}+\dfrac{7}{9}-\dfrac{5}{6}=\dfrac{83}{126}\)

5: \(=\dfrac{-5-2}{7}+\dfrac{3+1}{4}-\dfrac{1}{5}=-\dfrac{1}{5}\)

6: \(=\dfrac{-3-28}{31}+\dfrac{-6-1}{17}+\dfrac{1-5}{25}=-1-\dfrac{7}{17}-\dfrac{4}{25}=-\dfrac{668}{425}\)

4. \(\dfrac{2}{7}+\dfrac{1}{9}+\dfrac{3}{7}+\dfrac{5}{9}+\dfrac{-5}{6}\)

\(=\left(\dfrac{2}{7}+\dfrac{3}{7}\right)+\left(\dfrac{1}{9}+\dfrac{5}{9}\right)+\dfrac{-5}{6}\)

\(=\dfrac{5}{7}+\dfrac{2}{3}+\dfrac{-5}{6}\)

\(=\dfrac{30+28+\left(-35\right)}{42}=\dfrac{23}{42}\)
 

5. \(\dfrac{-5}{7}+\dfrac{3}{4}+\dfrac{-1}{5}+\dfrac{-2}{7}+\dfrac{1}{4}\)

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

\(=\dfrac{-7}{7}+\dfrac{4}{4}+\dfrac{-1}{5}\)

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

\(=-\dfrac{1}{5}\)
 

6. \(\dfrac{-3}{31}+\dfrac{-6}{17}+\dfrac{1}{25}+\dfrac{-28}{31}+\dfrac{-1}{17}+\dfrac{-1}{5}\)

\(=\left(\dfrac{-3}{31}+\dfrac{-28}{31}\right)+\left(\dfrac{-6}{17}+\dfrac{-1}{17}\right)+\dfrac{1}{25}+\dfrac{-1}{5}\)

\(=\dfrac{-31}{31}+\dfrac{-7}{17}+\dfrac{1}{25}+\dfrac{-1}{5}\)

\(=-1+\dfrac{-7}{17}+\dfrac{1}{25}+\dfrac{-1}{5}\)

\(=\dfrac{-425+\left(-175\right)+17+\left(-85\right)}{425}=\dfrac{-668}{425}\)

1. Sai vì 6 x 6 x 46 x 56 kết quả luôn có chử số tận cùng là 6. Mà 8 - 6 = 2 => ĐPCM

2. Sai nốt :) Vì không có 2 số nào nhân lại có chữ số tận cùng = 7 => ĐPCM

3. CMM, sai hoài vậy =)) 11 x 21 x 31 x 41 có chữ số tận cùng = 1. 19 x 25 x 37 có chữ số tận cùng = 5. 1 - 5 = 6 => ĐPCM

a, sai

b, sai

c, đúng

Câu a: Sai

Câu b:Sai

Câu c: Đúng

a, sai

b, sai

c, đúng

Giải;

1) \(347.222-222.\left(216+184\right):8\) 

\(=347.222-222.400:8\) 

\(=347.222-222.50\) 

\(=222.\left(347-50\right)\) 

\(=222.297\)

\(=65934\) 

2) \(132-\left[116-\left(132-128\right).22\right]\) 

\(=132-\left[116-4.22\right]\) 

\(=132-\left[116-88\right]\) 

\(=132-28\) 

\(=104\) 

3) \(16:\left\{400:\left[200-\left(37+46.3\right)\right]\right\}\) 

\(=16:\left\{400:\left[200-\left(37+138\right)\right]\right\}\) 

\(=16:\left\{400:\left[200-175\right]\right\}\) 

\(=16:\left\{400:25\right\}\) 

\(=16:16\) 

\(=1\) 

4) \(\left\{184:\left[96-124:31\right]-2\right\}.3651\) 

\(=\left\{184:\left[96-4\right]-2\right\}.3651\) 

\(=\left\{184:92-2\right\}.3651\) 

\(=\left\{2-2\right\}.3651\) 

\(=0.3651\) 

\(=0\) 

5) \(46-\left[\left(16+71.4\right):15\right]-2\) 

\(=46-\left[\left(16+284\right):15\right]-2\) 

\(=46-\left[300:15\right]-2\)

 \(=46-20-2\) 

\(=24\)

6) \(3^3.18+72.4^2-41.18\) 

\(=18.\left(27-41\right)+72.16\) 

\(=18.-14+1152\) 

\(=-252+1152\) 

\(=900\)

Giải: (tiếp)

7) \(\left(56.46-25.23\right):23\) 

\(=\left(2576-575\right):23\) 

\(=2001:23\) 

\(=87\) 

8) \(\left(28.54+56.36\right):21:2\) 

\(=\left(1512+2016\right):21:2\) 

\(=3528:21:2\) 

\(=84\) 

9) \(\left(76.34-19.64\right):\left(38.9\right)\) 

\(=\left(2584-1216\right):342\) 

\(=1368:342\) 

\(=4\) 

10) \(\left(2+4+6+...+100\right).\left(36.333-108.111\right)\) 

\(=\left(2+4+6+...+100\right).\left(11988-11988\right)\) 

\(=\left(2+4+6+...+100\right).0\) 

\(=0\) 

11) \(\left(5.4^{11}-3.16^5\right):4^{10}\) 

\(=5.4^{11}:4^{10}-3.16^5:4^{10}\) 

\(=5.4-3.1\) 

\(=20-3\) 

\(=17\) 

12) \(7256.4375-725:3650+4375.7255\) 

\(=4375.\left(7256+7255\right)-\dfrac{29}{146}\) 

\(=4375.14511-\dfrac{29}{146}\) 

\(=63485624,8\) 

Câu 12 ko chắc!