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Have looked in the user guide - cannot find information on use of the quadratic formula.

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Just use the square root button...? – Hans Lundmark Nov 14 '11 at 11:49

Most likely, the Casio fx-85GT PLUS (as with virtually all scientific calculators) doesn't do anything special for the quadratic formula. If you know the quadratic formula, you can substitute the values of the coefficients of your quadratic and then enter the resulting expressions into your calculator.

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Looking through the manual (via Google), there are two very unusual features:

(1) It can convert between a fraction and a repeated decimal, with dots on top of the first and last repeated digits.

(2) It can find the prime factors of integers up to $10^{10}$.

It can also create tables of function values and do quadratic (as well as linear) regression. Perhaps you can combine these to solve a quadratic (just as you can intersection of two lines using linear regression).

However, it does not seem to be able to directly solve a quadratic equation.

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