1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

= (1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)

=   20   +   20   +   20   +   20  +   20    +   20  +   20   +    20   +   20  +30

=  20x9 +30

=180+30

=210

tk cho mik nhé

\(\frac{\left(20+1\right)x20}{2}\)

=(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+10

=20+20+20+20+20+20+20+20+20+10

=190

55.tk mk đc ko mk kb ?

1+2+3+4+5+6+7+8+9+10= 55

kb vs mk nha

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55

=1

=4

=9

=16

=25

=36

=42

=64

=81

=100

1

4

9

16

25

36

49

64

81

100

= 1

= 4

= 9

= 16

= 25

= 36

= 49

= 64

= 81

= 100

1+ 2 × 3 + 4 × 5 + 6 × 7 + 8 × 9

= 1 + 6 + 20 + 42 + 81

= 7 + 20 + 42 + 81

= 72 + 42 + 81

= 114 + 81

= 195

=569

ket ban nha

1+ 6+20+42+72 

= 771

1+2+3+4+5+6+7+8+9+10=55 nha

hok tốt ^_^

Trả lời :

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10

= ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 10 + 5

= 10 + 10 + 10 + 10 + 10 + 5

= 10 x 5 + 5

= 50 + 5

= 55

~Moon~

1+2+3+4+5+6+7+8+9+10

= { 1+9 } + { 2+8 } + { 3+7 } + { 4+6 } +5 +10

= 10       +  10      +   10      +   10      +  15

= 55

1+2+3+4+5+6+7+8+9+10

=(1+9)+(2+8)+(3+7)+(4+6)+5+10

=10+10+10+10+10+5

=50+5

=55

k mk nha. Mk k bạn cho nha

Tổng bằng  55

OK  mk kb rồi đấy

=55 bn nhé

kết quả bằng 45 nhé bạn, nhá, mình kết bạn với bạn rồi đó

45 ban nhe

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45.

Đúng 100% luôn!

Ai tk cho mình mình tk lại.

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20

=(20+1)+(2+19)+...+(10+11) có 10 cặp số

=21+21+..+21 Có 10 số hạng

=21 x 10

=210

(20+1).20:2=210

tích

\(=\frac{20\left(21\right)}{2}=210\)

\(a,=\frac{7-1}{1.3.7}+\frac{9-3}{3.7.9}+\frac{13-7}{7.9.13}+\frac{15-9}{9.13.15}\)\(+\frac{19-13}{13.15.19}\)

\(=\frac{1}{1.3}-\frac{1}{3.7}+\frac{1}{3.7}-\frac{1}{7.9}+\frac{1}{7.9}-\frac{1}{9.13}+\frac{1}{9.13}-\frac{1}{13.15}+\frac{1}{13.15}-\frac{1}{15.19}\)

\(=\frac{1}{1.3}-\frac{1}{15.19}=\frac{95}{285}-\frac{1}{285}=\frac{94}{285}\)

\(b,=\frac{1}{6}.\left(\frac{6}{1.3.7}+\frac{6}{3.7.9}+\frac{6}{7.9.13}+\frac{6}{9.13.15}+\frac{6}{13.15.19}\right)\)

làm giống như trên

\(c,=\frac{1}{8}.\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+\frac{5-3}{3.4.5}+...+\frac{50-48}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{48.49}-\frac{1}{49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{2}-\frac{1}{2450}\right)=\frac{1}{16}.\left(\frac{1225}{2450}-\frac{1}{2450}\right)=\frac{153}{4900}\)

\(d,=\frac{5}{7}.\left(\frac{7}{1.5.8}+\frac{7}{5.8.12}+\frac{7}{8.12.15}+...+\frac{7}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{8-1}{1.5.8}+\frac{12-5}{5.8.12}+\frac{15-8}{8.12.15}+...+\frac{40-33}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{1.5}-\frac{1}{5.8}+\frac{1}{5.8}-\frac{1}{8.12}+\frac{1}{8.12}-\frac{1}{12.15}+...+\frac{1}{33.36}-\frac{1}{36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{5}-\frac{1}{1440}\right)=\frac{5}{7}.\left(\frac{288}{1440}-\frac{1}{1440}\right)=\frac{41}{288}\)

P/S: . là nhân nha

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