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 #2
avatar+26400 
+15

How many numbers between 1000 and 2000 leave a remainder of 3 when divided by 11 ?

 

$$\small{
\begin{array}{rcl}
1. & 1004 & \text{remainder~} 3 \\
2. & 1015 & \text{remainder~} 3\\
3. & 1026 & \text{remainder~} 3\\
4. & 1037 & \text{remainder~} 3\\
5. & 1048 & \text{remainder~} 3\\
6. & 1059 & \text{remainder~} 3\\
7. & 1070 & \text{remainder~} 3 \\
8. & 1081 & \text{remainder~} 3\\
9. & 1092 & \text{remainder~} 3\\
10. & 1103 & \text{remainder~} 3\\
11. & 1114 & \text{remainder~} 3\\
12. & 1125 & \text{remainder~} 3\\
13. & 1136 & \text{remainder~} 3\\
14. & 1147 & \text{remainder~} 3\\
15. & 1158 & \text{remainder~} 3\\
16. & 1169 & \text{remainder~} 3\\
17. & 1180 & \text{remainder~} 3\\
18. & 1191 & \text{remainder~} 3\\
19. & 1202 & \text{remainder~} 3\\
20. & 1213 & \text{remainder~} 3\\
21. & 1224 & \text{remainder~} 3\\
22. & 1235 & \text{remainder~} 3\\
23. & 1246 & \text{remainder~} 3\\
24. & 1257 & \text{remainder~} 3\\
25. & 1268 & \text{remainder~} 3\\
26. & 1279 & \text{remainder~} 3\\
27. & 1290 & \text{remainder~} 3\\
28. & 1301 & \text{remainder~} 3\\
29. & 1312 & \text{remainder~} 3\\
30. & 1323 & \text{remainder~} 3\\
31. & 1334 & \text{remainder~} 3\\
32. & 1345 & \text{remainder~} 3\\
33. & 1356 & \text{remainder~} 3\\
34. & 1367 & \text{remainder~} 3\\
35. & 1378 & \text{remainder~} 3\\
36. & 1389 & \text{remainder~} 3\\
37. & 1400 & \text{remainder~} 3\\
38. & 1411 & \text{remainder~} 3\\
39. & 1422 & \text{remainder~} 3\\
40. & 1433 & \text{remainder~} 3\\
41. & 1444 & \text{remainder~} 3\\
42. & 1455 & \text{remainder~} 3\\
43. & 1466 & \text{remainder~} 3\\
44. & 1477 & \text{remainder~} 3\\
45. & 1488 & \text{remainder~} 3\\
46. & 1499 & \text{remainder~} 3\\
47. & 1510 & \text{remainder~} 3\\
48. & 1521 & \text{remainder~} 3\\
49. & 1532 & \text{remainder~} 3\\
50. & 1543 & \text{remainder~} 3\\
51. & 1554 & \text{remainder~} 3\\
52. & 1565 & \text{remainder~} 3\\
53. & 1576 & \text{remainder~} 3\\
54. & 1587 & \text{remainder~} 3\\
55. & 1598 & \text{remainder~} 3\\
56. & 1609 & \text{remainder~} 3\\
57. & 1620 & \text{remainder~} 3\\
58. & 1631 & \text{remainder~} 3\\
59. & 1642 & \text{remainder~} 3\\
60. & 1653 & \text{remainder~} 3\\
61. & 1664 & \text{remainder~} 3\\
62. & 1675 & \text{remainder~} 3\\
63. & 1686 & \text{remainder~} 3\\
64. & 1697 & \text{remainder~} 3\\
65. & 1708 & \text{remainder~} 3\\
66. & 1719 & \text{remainder~} 3\\
67. & 1730 & \text{remainder~} 3\\
68. & 1741 & \text{remainder~} 3\\
69. & 1752 & \text{remainder~} 3\\
70. & 1763 & \text{remainder~} 3\\
71. & 1774 & \text{remainder~} 3\\
72. & 1785 & \text{remainder~} 3\\
73. & 1796 & \text{remainder~} 3\\
74. & 1807 & \text{remainder~} 3\\
75. & 1818 & \text{remainder~} 3\\
76. & 1829 & \text{remainder~} 3\\
77. & 1840 & \text{remainder~} 3\\
78. & 1851 & \text{remainder~} 3\\
79. & 1862 & \text{remainder~} 3\\
80. & 1873 & \text{remainder~} 3\\
81. & 1884 & \text{remainder~} 3\\
82. & 1895 & \text{remainder~} 3\\
83. & 1906 & \text{remainder~} 3\\
84. & 1917 & \text{remainder~} 3\\
85. & 1928 & \text{remainder~} 3\\
86. & 1939 & \text{remainder~} 3\\
87. & 1950 & \text{remainder~} 3\\
88. & 1961 & \text{remainder~} 3\\
89. & 1972 & \text{remainder~} 3\\
90. & 1983 & \text{remainder~} 3\\
91. & 1994 & \text{remainder~} 3\\
\end{array}
}$$

 

Jun 23, 2015
 #1
avatar+33661 
+13
Jun 23, 2015
 #2
avatar+118723 
+8

 

Now it is right     I had just made a careless error right at the beginning.

Thanks for spotting it Alan     [ I do have a question at the bottom though]

 

the correct answer is   

I have not found my errors  :(

 

---------------------------------------------------------------------------------

 

$$\\\int\frac{(2cos3x + 3sinx) }{sin^3x }dx\\\\
=\int\frac{2(cos2xcosx-sin2xsinx) + 3sinx }{sin^3x }dx\\\\
=\int\frac{2cos2xcosx - 2sin2xsinx) + 3sinx }{sin^3x }dx\\\\
=\int\frac{2(cos^2x-sin^2x)cosx - 2(2sinxcosx)sinx) + 3sinx }{sin^3x }dx\\\\
=\int\frac{2cosxcos^2x-2cosxsin^2x - 4sin^2xcosx + 3sinx }{sin^3x }dx\\\\
=\int\frac{2cos^3x-2cosxsin^2x - 4sin^2xcosx + 3sinx }{sin^3x }dx\\\\
=\int\frac{2cos^3x - 6sin^2xcosx + 3sinx }{sin^3x }dx\\\\
=\int\;2cot^3x-\frac{6cosx}{sinx} + 3csc^2x \;dx\\\\
=\int\;2cot^3x\;dx -\int\;\frac{6cosx}{sinx}\;dx +\int\; 3csc^2x \;dx\\\\
=\int\;2cot^3x\;dx -6ln(sinx) +\int\; 3csc^2x \;dx\\\\
=-6ln(sinx)\;+\;\int\;2cot^3x\;dx -3cotx\\\\
=-6ln(sinx)\;-3cotx\;+\;\int\;2cot^3x\;dx$$

 

$$\\$NOW I'll use the reduction formula$\\\\
\int\;2cot^3x\;dx\\\\
=2[\frac{-Cot^{3-1}x}{3-1}-\int\;cot^{-2+3}x\;dx]\\\\
=2[\frac{-Cot^{2}x}{2}-\int\;cotx\;dx]\\\\
=-Cot^{2}x-2\;ln(sinx)\\\\$$

 

$$\\$So - continuing from before$\\\\
=-6ln(sinx)\;-3cotx\;+\;\int\;2cot^3x\;dx \\\\
=-6ln(sinx)\;-3cotx\;+\; -Cot^{2}x-2\;ln(sinx) +c \\\\
=-8ln(sinx)\;-3Cotx-Cot^{2}x \;+c \\\\$$

-------------------------------------------------------------

Now this is correct but Wolfram Alpha is telling me that  $$cot^2x=csc^2x$$   for the restricted values involed here.

WHY IS THAT ?

Jun 23, 2015
 #33
 #32
 #31

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