why is 4/3 πr^3 a monomial  

Find a polynomial f(x) of degree 5 that has the following zeros. 
8,7,0,1,-5

Relative Frequency is useful when comparing the absolute frequency results within
the data set.

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A typical vitamin C tablet (containing pure ascorbic acid, H2C6H6O6) weighs 500. mg. One vitamin C tablet is dissolved in enough water to make 200.0 mL of solution. calculate the pH of this solution. Ascorbic acid is a diprotic acid.

How can I solve this differental equation?
$\begin{array}{rl}(\frac{1}{x}-y^2\frac{1}{(x-y{)}^2})dx& +(x^2\frac{1}{(x-y{)}^2}-\frac{1}{y})dy=0?\end{array}$
I have this as a part of my homework, and there were like 20 other differential equations that I easily solved, but this one stood out.
This is not linear, not separable. I don't know how to approach this problem. If you could tell me to what class this kind of equation belongs, and some methods to solve them, I would be very very glad.

A normal distribution has a mean of 80;and a standard deviation of 12. Determine the value below which 95 percent of the observations will occur.