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a; \(\dfrac{1}{4}\) + \(\dfrac{2}{5}\) + \(\dfrac{6}{8}\) + \(\dfrac{9}{15}\) + \(\dfrac{8}{1}\)
= (\(\dfrac{1}{4}\) + \(\dfrac{6}{8}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{9}{15}\)) + \(\dfrac{8}{1}\)
= (\(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{3}{5}\)) + 8
= 1 + 1 + 8
= 2 + 8
= 10
b; \(\dfrac{1}{2}\) + \(\dfrac{2}{4}\) + \(\dfrac{3}{6}\) + \(\dfrac{4}{8}\) + \(\dfrac{5}{10}\) + \(\dfrac{6}{12}\) + \(\dfrac{7}{14}\) + \(\dfrac{8}{16}\) + \(\dfrac{10}{20}\)
= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x (\(\dfrac{2}{2}\) + \(\dfrac{3}{3}\) + \(\dfrac{4}{4}\) + \(\dfrac{5}{5}\)+ \(\dfrac{6}{6}+\dfrac{7}{7}+\dfrac{8}{8}\) + \(\dfrac{10}{10}\))
= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x (1 + 1 +1 + 1+ 1+ 1+ 1 +1)
= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) x 1 x 8
= \(\dfrac{1}{2}\) + \(\)\(\dfrac{1}{2}\) x 8
= \(\dfrac{1}{2}\) + 4
= \(\dfrac{9}{2}\)
a; \(\dfrac{1}{4}\) + \(\dfrac{2}{5}\) + \(\dfrac{6}{8}\) + \(\dfrac{9}{15}\) + \(\dfrac{8}{1}\)
= (\(\dfrac{1}{4}\) + \(\dfrac{6}{8}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{9}{15}\)) + 8
= (\(\dfrac{1}{4}\) + \(\dfrac{3}{4}\)) + (\(\dfrac{2}{5}\) + \(\dfrac{3}{5}\)) + 8
= 1 + 1 + 8
= 2 + 8
= 10
b; \(\dfrac{1}{2}\) + \(\dfrac{2}{4}\) + \(\dfrac{3}{6}\) + \(\dfrac{4}{8}\) + \(\dfrac{5}{10}\) + \(\dfrac{6}{12}\) + \(\dfrac{7}{14}\) + \(\dfrac{8}{16}\) + \(\dfrac{9}{18}\) + \(\dfrac{10}{20}\)
= \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\)
= \(\dfrac{1}{2}\) x 10
= 5
2 + 4 + 6 + 8 + 10 + 12 + 14 + 16 + 18 + 20
= ( 2 + 20 ) + ( 4 + 18 ) + ( 6 + 16 ) + ( 8 + 14 ) + ( 10 +12 )
= 22 + 22 + 22 + 22 + 22
= 22 x 5
= 110
Bài a,b tự làm:
c,99 x 2014-50 x 2014+2014
=(99-50+1) x 2014
=50 x 2014
=100700
d,(81 x 11):9
=891:9
=99
e,46 x 6+46 x 4
=46 x (4+6)
=46 x 10
=460
1+1+2+2+3+3+4+4+5+5+6+6+7+7+8+9+8+9+10+12+13+14+15+16+17+18+19+20+30+1000000=10000274
B=(-2+4)+(6-8)+(10-12)+...+(2010-2012)
B=-2+(-2)+(-2)+...+(-2)
B=-2.1006
B=-2012
Vậy B=-2012
\(2+4+6+8+10+12+14+16+18+20+22\)
\(=\left(2+22\right)+\left(4+20\right)+\left(6+18\right)+\left(8+16\right)+\left(10+14\right)+12\)
\(=24+24+24+24+24+12\)
\(=24\times5+12\)
\(=12\times2\times5+12\)
\(=12\times\left(2\times5+1\right)\)
\(=12\times11=121\)
a) 37 x 18 - 9 x 54 + 100
= 37 x 2 x 9 - 9 x 54 + 100
= 74 x 9 - 8 x 54 + 100
= ( 74 - 54 ) x 9 + 100
= 20 x 9 + 20 x 5
= 20 x ( 9 + 5 )
= 20 x 14
= 280
Dãy số đó có số số hạng la :
( 20 - 2 ) : 2 + 1 = 10 ( số )
Tổng của dãy số hạng đó là :
( 20 + 2 ) x 10 : 2 = 110
Đáp số 110
\(B=\left(2+18\right)+\left(4+16\right)+\left(6+14\right)+\left(8+12\right)+20+10\)
\(=20+20+20+20+20+10\)
\(=20x5+10=100+10=110\)