Exercise algebra.equations.quadratic.no-abc
Description
solve a quadratic equation without abc-formula
Derivation
![](http://ideas.cs.uu.nl/images/external.png)
x-1 == 10 or x-1 == -10
⇒ algebra.equations.coverup.onevar.minus-leftx == 11 or x-1 == -10
⇒ algebra.equations.coverup.onevar.minus-leftx == 11 or x == -9