difference between solve and nsolve ?.

[alexsunny123]

Hello,


Well I locate the problem that I was trying to solve: it is a problem of pH when the solution is very diluted, we find the equation
x ^ 2-1.2e-7.x-1e-14 = 0
hop I enter my nspire and I paste that in solve ... which gives me a nonsense in answer x = 6e-8 (and this without second solution)
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I remember that there is nsolve and I start again: only one result and negative.

I had to do nsolve (x ^ 2-1.2e-7.x-1e-14 = 0, x) | x0 to finally get the correct answer.

is it normal that solve sent me to graze?


thanks
alexsunny

[Bisam]

It is quite normal that both solve and nsolve can't get the solution right because the errors made when searching and truncating are soon bigger than the solution itself.
As you found, nsolve has an option allowing to look for a solution near a given attempt, and doing so gives best chances of finding right.

solve tries to find all solutions... but can't correct evaluation errors made during computation. Here, one workaroudn would be to replace numbers by letters, solve exactly with letters and then replace letters by numbers (using the "where" bar

|

).