How do you set up the quadratic formula with the correct numbers? x² - 4x - 15 = 0

2 Answers
Jumbotron
Jun 7, 2018

#x = (4 + sqrt(76))/2 or (4 - sqrt(76))/2#

Explanation:

#x^2 - 4x - 15 = 0#

Recall;

#ax^2 + bx + c = 0#

Comparing you will have;

#a = 1#

#b = -4#

#c = -15#

Using Quadratic Formula;

#x = (-b +- sqrt(b^2 - 4ac))/(2a)#

Plugging the values into the equation..

#x = (-(-4) +- sqrt((-4)^2 - 4(1)(-15)))/(2(1))#

#x = (4 +- sqrt(16 + 60))/2#

#x = (4 +- sqrt(76))/2#

#x = (4 + sqrt(76))/2 or (4 - sqrt(76))/2#

Jim G.
Jun 7, 2018

#"see explanation"#

Explanation:

#"the solution of a quadratic equation in "color(blue)"standard form"#

#•color(white)(x)ax^2+bx+c=0;a!=0#

#"can be solved using the "color(blue)"quadratic formula"#

#•color(white)(x)x=(-b+-sqrt(b^2-4ac))/(2a)#

#x^2-4x-15=0" is in standard form"#

#"with "a=1,b=-4" and "c=-15#

#x=(-(-4)+-sqrt((-4)^2-(4xx1xx-15)))/(2xx1)#

#color(white)(x)=(4+-sqrt(16+60))/2#

#color(white)(x)=(4+-sqrt76)/2=(4+-2sqrt19)/2=2+-sqrt19#

#x~~-2.36" or "x~~6.36" to 2 dec. places"#

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