viết số thứ tự và số đếm từ 1đến 100
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viết liên tiếp từ 1 đến 15 ta được số :
\(123456789101112131415\)
khi xóa đi 10 chữ số thì số lớn nhất thu được là
\(91112131415\)( xóa chữ số ở vị trí 1,2,3,4,5,6,7,8,10,11)
Số nhỏ nhất thu được là :
\(10111131415\)( xóa chữ số ở vị trí 2,3,4,5,6,7,8,9,10,15)
từ 1 tới 9 có 9 số có 1 chữ số
từ 10 tới 99 có 90 số có hai chữ số
100 có 3 chữ số
vậy cần 9 + (90x2)+3=202 chũ số
1 . Từ 1 đến 99 có số số hạng là : ( 99 - 1 ) : 1 + 1 = 99 ( số )
Tổng là : ( 99 + 1 ) . 99 : 2 = 4950 .
2. cách 1
M = { 11 ; 13 ; 15 ; 17 ; 19 }
cách 2
M = { x thuộc N* / x là các số lẻ / 10 < x < 20 }
1.
tổng các số tự nhiên từ 1 đến 99 là:
\(\frac{\left(99+1\right).99}{2}\)=4950
2. C1: gọi các số tự nhiên là x:
M={x thuộc/10<x<20}
C2: M={11;12;13;14;15;16;17;18;19}
1 - 10
1 (one) → 1st (first)
2 (two) → 2nd (second)
3 (three) → 3rd (third)
4 (four) → 4th (fourth)
5 (five) → 5th (fifth)
6 (six) → 6th (sixth)
7 (seven) → 7th (seventh)
8 (eight) → 8th (eighth)
9 (nine) → 9th (ninth)
10 (ten) → 10th (tenth)
11 - 20
11 (eleven) → 11th (eleventh)
12 (twelve) → 12th (twelfth)
13 (thirteen) → 13th (thirteenth)
14 (fourteen) → 14th (fourteenth)
15 (fifteen) → 15th (fifteenth)
16 (sixteen) → 16th (sixteenth)
17 (seventeen) → 17th (seventeenth)
18 (eighteen) → 18th (eighteenth)
19 (nineteen) → 19th (nineteenth)
20 (twenty) → 20th (twentieth)
21 - 30
21 (twenty-one) → 21st (twenty-first)
22 (twenty-two) → 22nd (twenty-second)
23 (twenty-three) → 23rd (twenty-third)
24 (twenty-four) → 24th (twenty-fourth)
25 (twenty-five) → 25th (twenty-fifth)
26 (twenty-six) → 26th (twenty-sixth)
27 (twenty-seven) → 27th (twenty-seventh)
28 (twenty-eight) → 28th (twenty-eighth)
29 (twenty-nine) → 29th (twenty-ninth)
30 (thirty) → 30th (thirtieth)
31 - 40
31 (thirty-one) → 31st (thirty-first)
32 (thirty-two) → 32nd (thirty-second)
33 (thirty-three) → 33rd (thirty-third)
34 (thirty-four) → 34th (thirty-fourth)
35 (thirty-five) → 35th (thirty-fifth)
36 (thirty-six) → 36th (thirty-sixth)
37 (thirty-seven) → 37th (thirty-seventh)
38 (thirty-eight) → 38th (thirty-eighth)
39 (thirty-nine) → 39th (thirty-ninth)
40 (forty) → 40th (fourtieth)
41 - 50
41 (forty-one) → 41st (forty-first)
42 (forty-two) → 42nd (forty-second)
43 (forty-three) → 43rd (forty-third)
44 (forty-four) → 44th (forty-fourth)
45 (forty-five) → 45th (forty-fifth)
46 (forty-six) → 46th (forty-sixth)
47 (forty-seven) → 47th (forty-seventh)
48 (forty-eight) → 48th (forty-eighth)
49 (forty-nine) → 49th (forty-ninth)
50 (fifty) → 50th (fiftieth)
51 - 60
51 (fifty-one) → 51st (fifty-first)
52 (fifty-two) → 51nd (fifty-second)
53 (fifty-three) → 53rd (fifty-third)
54 (fifty-four) → 54th (fifty-fourth)
55 (fifty-five) → 55th (fifty-fifth)
56 (fifty-six) → 56th (fifty-sixth)
57 (fifty-seven) → 57th (fifty-seventh)
58 (fifty-eight) → 58th (fifty-eighth)
59 (fifty-nine) → 59th (fifty-ninth)
60 (sixty) → 60th (sixtieth)
61 - 70
61 (sixty-one) → 61st (sixty-first)
62 (sixty-two) → 61nd (sixty-second)
63 (sixty-three) → 63rd (sixty-third)
64 (sixty-four) → 64th (sixty-fourth)
65 (sixty-five) → 65th (sixty-fifth)
66 (sixty-six) → 66th (sixty-sixth)
67 (sixty-seven) → 67th (sixty-seventh)
68 (sixty-eight) → 68th (sixty-eighth)
69 (sixty-nine) → 69th (sixty-ninth)
70 (seventy) → 70th (seventieth)
71 - 80
71 (seventy-one) → 71st (seventy-first)
72 (seventy-two) → 72nd (seventy-second)
73 (seventy-three) → 73rd (seventy-third)
74 (seventy-four) → 74th (seventy-fourth)
75 (seventy-five) → 75th (seventy-fifth)
76 (seventy-six) → 76th (seventy-sixth)
77 (seventy-seven) → 77th (seventy-seventh)
78 (seventy-eight) → 78th (seventy-eighth)
79 (seventy-nine) → 79th (seventy-ninth)
80 (eighty) → 80th (eightieth)
81 - 90
81 (eighty-one) → 81st (eighty-first)
82 (eighty-two) → 82nd (eighty-second)
83 (eighty-three) → 83rd (eighty-third)
84 (eighty-four) → 84th (eighty-fourth)
85 (eighty-five) → 85th (eighty-fifth)
86 (eighty-six) → 86th (eighty-sixth)
87 (eighty-seven) → 87th (eighty-seventh)
88 (eighty-eight) → 88th (eighty-eighth)
89 (eighty-nine) → 89th (eighty-ninth)
90 (ninety) → 90th (ninetieth)
91 - 100
91 (ninety-one) → 91st (ninety-first)
92 (ninety-two) → 92nd (ninety-second)
93 (ninety-three) → 93rd (ninety-third)
94 (ninety-four) → 94th (ninety-fourth)
95 (ninety-five) → 95th (ninety-fifth)
96 (ninety-six) → 96th (ninety-sixth)
97 (ninety-seven) → 97th (ninety-seventh)
98 (ninety-eight) → 98th (ninety-eighth)
99 (ninety-nine) → 99th (ninety-ninth)
100 (one hundred) → 100th (one hundredth)
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