Plotting Quadratic Equation

[Luis Pecht]

Hi, My son is learning Quadratic Equation and I am trying using Stata to help him.

But before help him, First, I need help myself.

Suppose we want to solve the following equation and Bhaskara's is here to help us on that

The equation -5x² + 240x + 5000 = 0 has 2 real roots when solved:

. di ((-240)-((240)^2-4*(-5)*5000)^(1/2))/(2*(-5))
63.698866

. di ((-240)+((240)^2-4*(-5)*5000)^(1/2))/(2*(-5))
-15.698866

double checked with : https://coolconversion.com/math/quad...ion-calculator

however when plotting using:

twoway (function (-(5*x)^2)+(240*x)+5000, range(-20 20)),yline(0) xline(0)

the plot crosses the X axis in ~-10 and ~+20, when Y=0





Am I using the appropriate function to plot it ?

[Joseph Coveney]

Am I using the appropriate function to plot it ?

This works for me:

mata: st_matrix("Roots", Re(polyroots( (5000, 240, -5) )))
graph twoway function y = -5 * x * x + 240 * x + 5000, range(-20 70) ///
    yline(0) xline(`=Roots[1, 1]' `=Roots[1, 2]')

[Luis Pecht]

great, Joseph!! thks.

[Clyde Schechter]

Your graph command has a different function:

twoway (function (-(5*x)^2)+(240*x)+5000, range(-20 20)),yline(0) xline(0)

//  should be 
 twoway (function (-5*x^2)+(240*x)+5000, range(-20 20)),yline(0) xline(0)

By putting the (-5*x) in parentheses, you were squaring the -5, so you had a quadratic coefficient of 25 instead of -5. The fact that the graph showed an inverted parabola instead of an upright one is a clue that it was showing you the wrong function.

[Luis Pecht]

Yes, Prof Clyde, Joseph simplify things in #2, using:

twoway (function (-5*x*x)+(240*x)+5000, range(-20 70)),yline(0) xline(0)

thks,