Question 226036
Use algebra so solve (x-1)(x+2)=18 


Step 1.  Multiply out to get rid of the parenthesis.


{{{(x-1)(x+2)=x^2+2x-x-2=18}}}


{{{x^2+x-2=18}}}


Step 2.  Subtract 18 from both sides of the equation


{{{x^2+x-2-18=18-18}}}


{{{x^2+x-20=0}}}


{{{x^2+x-20=(x+5)(x-4)=0}}


Step 3.  This implies that {{{x+5=0}}} and {{{x-4=0}}} or {{{x=-5}}} and {{{x=4}}}


Step 4.   We can also solve using the quadratic equation shown below:


{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}}


where a=1, b=1 and c=-20


*[invoke quadratic "x", 1, 1, -20 ]


...same as before.


Step 5.  ANSWER:  The solutions are x=-5 and x=4.


I hope the above steps were helpful.


For FREE Step-By-Step videos in Introduction to Algebra, please visit http://www.FreedomUniversity.TV/courses/IntroAlgebra and for Trigonometry visit http://www.FreedomUniversity.TV/courses/Trigonometry.


Good luck in your studies!


Respectfully,
Dr J