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目前顯示的是 9月 10, 2018的文章

Can I use a fraction of vector to reverse cross product?

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Clash Royale CLAN TAG #URR8PPP up vote 2 down vote favorite The cross product is defined by the formula $$ vec A_i times vec B_j = |A|,|B| sinalpha = vec C_k tag1$$ Where $alpha$ is angle between vectors $A$ and $B$. $C$ is a pseudovector perpendicular to $A$ and $B$. And i, j, k is standard basis direction mutually perpendicular to each other. Assuming $alpha$ it equals $90$ degrees then $sin90=1$. This discussion is only for this case. Reverse cross product we encounter for a problem $$ vec C_k times vec A_i = vec B`_j tag2$$ The $B`$ vector has the correct direction but its value usually not equal $B$, because use $(1)$ $$ left(|A| , |B|right)_k times vec A_i =|A|^2 , |B| = vec B`_j tag3$$ but using the proportion $(1)$ we can easily reverse them $$AB=C rightarrow B=Cover A tag4$$ If we find the inverse of the vector $A$, we can in this case when $alpha=90$ degrees reverse cross product. How to get a vector $1/A$? The value of the vector is $$|A|= sqrta_x^2 + a_y ^2 +

Number of integer solutions combinatorics problem

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Clash Royale CLAN TAG #URR8PPP up vote 3 down vote favorite 1 what is the number of integer solutions to $$x_1+x_2+x_3+x_4+x_5=18$$ with $$x_1ge1;;;x_2ge2;;;x_3ge3;;;x_4ge4;;; x_5ge5$$ I know I have to use this formula $$frac(n+r-1)!(n-1)!;r!= n+r-1choose r$$ My instinct says that I should use $n=18-1-2-3-4=18-15=3$ and $r=5$ but I m not sure it makes sense? Anyone can help me please? combinatorics share | cite | improve this question edited Aug 28 at 18:24 Davide Morgante 2,550 6 23 asked Aug 28 at 18:20 voldetort 16 1 4 You can use standard methods if you replace $x_i$ with $y_i=x_i-i$. Then, since $1+2+3+4+5=15$ we just have $sum y_i=3$ and $y_i≥0$. – lulu Aug 28 at 18:26 Well ok but can you explain it a bit better? what is the standard method I should use and how does it help me get to a conclusion – voldetort Aug 28 at 18:28 1 The standard method goes by the name Stars and Bars. Th