First, define an implicit function:
In general, if in the equation F (x, y) = 0, x is any value in a taking zone, accordingly there is always present a unique y-value satisfying this equation, the said equation F (x, y) = 0 is determined in an implicit function of the interval.
That is: In the equation F (x, y) = 0, the determination can be a function of x y is an implicit function (function of two variables x refers to a change in the process, y, for each of a range of x a value, y are determined and its corresponding value, y is a function of x)
Second, how derivative implicit function:
namely: (1) x derivation of equation
(2) in the equation y on x is considered as a formula, so the derivative of this equation x, i.e., the overall derivative of y, writing dy / dx or y '.
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1 
on both sides of the x-derivative:
Finally, since demand is y ', derived by sorting to formula.
2、
