Use the quadratic formula to solve the equation 4x^2 - 40x = 84?

Answer 1

4x^2-40x=84
or, 4x^2-40x-84=0
or, 4x^2-12x-28x-84=0
or, 4x(x-3)-28(x-3)=0
or, (x-3)(4x-28)=0
or, x = 3 and 4x=28
or, x = 3 and 7

there is a mistake of sign in your problem either it should be
4x^2+40x=84
or it should be 4x^2-40x=-84

in case you are sure ther the problem you gave is correct then use the following formula

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

where the equation is
ax^2+bx+c=0

from this we get the solution would be

x = [40 +/- sqrt (1600 + 1344)]/8
= 5 +/- 1/8 (54.25863986)
= 5 +/- 6.782329
= 11.782329 and - 1.782329

Answer 2

4x^2 - 40x = 84 subtract 84 from each side
4x^2-40x-84=0 divide by 4
x^2-10x-21=0
x=(10+/-√(100+84))/2
x=5+/-0.5√184
x=5+/-√46
x=5+√46, 5-√46

Answer 3

4x^2 - 40x = 84
4x^2 - 40x - 84 = 0
4(x^2 - 10x - 21) = 0
x^2 - 10x - 21 = 0

x = (-b ± sqrt(b^2 - 4ac))/(2a)

x = (-(-10) ± sqrt((-10)^2 - 4(1)(-21)))/(2(1))
x = (10 ± sqrt(100 + 84))/2
x = (10 ± sqrt(184))/2
x = (10 ± sqrt(4 * 46))/2
x = (10 ± 2sqrt(46))/2
x = 5 ± sqrt(46)

ANS : x = 5 ± sqrt(46)

Answer 4

Simplifying:
4(x² - 10x - 21)= 0
4(x - 3)(x-7) = 0

Answer 5

so it comes to 4x^2-40x-84=0

a=4
b=-40
c=-84

40+or-SQUARE ROOT of 1600-(4)(4)(-84)/8

40+-SQUARE ROOT of 1600+1344/8

40+- suare root of 2944 all divided by 8

Answer 6

I'm not putting all of the work, takes too long
X = 3, 7

Answer 7

np2000 has the right answer, 11.78 and -1.78

for the equation, check out http://mathworld.wolfram.com/QuadraticEquation.html

Answer 8

x= -1.78, 11.78