[Tutor] faulty code (maths)

[Roel Schroeven]

David schreef:
> Hello everybody,
> 
> I recently came across a book by Prof. Langtangen: Indroduction to 
> Computer Programming: http://folk.uio.no/hpl/INF1100/INF1100-ebook-Aug08.pdf
> 
> I am trying to solve exercise 1.18 ("Why does the following program not 
> work correctly?"), but I don't find the mistake: why does the line
> 
> q = sqrt(b*b - 4*a*c)
> 
> cause an error? I was playing around with the code, but got nowhere.
> 
> Here the entire code:
> 
> a = 2; b = 1; c = 2
> from math import sqrt
> q = sqrt(b*b - 4*a*c)
> x1 = (-b + q)/2*a
> x2 = (-b - q)/2*a
> print x1, x2
> 
> 
> Many thanks for a pointer!

Apart from the problem of taking the square root of a negative number, 
there is another problem in the code in the book: to calculate the roots 
of a * x**2 + b*x + c, there should be parentheses around 2*a in the 
calculation of x1 and x2:

x1 = (-b + q)/(2*a)
x2 = (-b - q)/(2*a)

Otherwise the code multiplies by a instead while it should instead 
divide by a.

-- 
The saddest aspect of life right now is that science gathers knowledge
faster than society gathers wisdom.
   -- Isaac Asimov

Roel Schroeven