Solve the quadratic equation: x^2 + 21 = 0?

Question

the answer is +/- (square root)7i, but I don't know how to get it

the answer is +/-(square root)7i, i don't know how to get it though, also what does "i" stand for and how do you get it??

Answer 1

"i" is used to represent the square root of -1. It doesn't have an actual value in the real number system, so i and multiples of i are called "imaginary numbers". They have their applications, but one thing they allow you to do is express the solution of any polynomial that normally doesn't have a (real) solution.

This one is easy enough to solve without any factoring or quadratic formula:

x^2 + 21 = 0
x^2 = -21
x = ±√(-21)
x = ±(√21)(√-1)
x = ±(√21)i

I don't know where you're getting ±(√7)i.

Answer 2

You probably can't get that answer because it's not the right answer at all.

±√7i would be the answer if your question was x^2 + 7 = 0

Answer 3

That's not the answer to the problem you've given.

x^2 + 21 = 0
x^2 = -21
x = (+/-) sqrt(-21) = (+/-) sqrt(21)*sqrt(-1) = (+/-)sqrt(21)i

i = sqrt(-1): a number which, when squared, equals -1. It's an imaginary number, hence the "i".

Answer 4

i = sqrt(-1)
x^2 + 21 = 0
x^2= -21
x = +/- sqrt(-21)
x = +/1 sqrt(21)*sqrt(-1)
x = +/- sqrt(21)i

+/- sqrt7i is incorrect totally.

Answer 5

use the quadratic formula

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

the i stands for an imaginary number, which means the number under the square root was negative, remember i = sqrt(-1)

Answer 6

As is, the problem answer is x=+/- sq rt -21

Answer 7

it is +/- sqrt(21)*i
i is the sqrt(-1)

Answer 8

Eleventy nine