Find the solutions to x⁴-5x²-36=0 and the x-intercepts of the graph of y=x⁴-5x²-36.

Question

asked Aug 13, 2020 103k views

1 Answer

Scott H · Answer 1 · 2020-08-19T22:48:47+0000

Answer:

The solutions
x^4-5x^2-36=0 are
$x=3,\:x=-3,\:x=2i,\:x=-2i$ and the x-intercepts of
y=x^4-5x^2-36 are
$x=3,\:x=-3$

Explanation:

Finding the solutions to
x^4-5x^2-36 means finding the roots, a root is where the function is equal to zero.

The x-intercept is the point at which the graph crosses the x-axis. At this point, the y-coordinate is zero.

To find the roots you need to:

Rewrite the equation with
u=x^2 and
u^2=x^4

u^2-5u-36=0

Solve by factoring

$\mathrm{Break\:the\:expression\:into\:groups}\\u^2-5u-36=\left(u^2+4u\right)+\left(-9u-36\right)$

$\mathrm{Factor\:out\:}u\mathrm{\:from\:}u^2+4u=u\left(u+4\right)$

$\mathrm{Factor\:out\:}-9\mathrm{\:from\:}-9u-36=-9\left(u+4\right)$

$u^2-5u-36=u\left(u+4\right)-9\left(u+4\right)$

$\mathrm{Factor\:out\:common\:term\:}u+4\\\left(u+4\right)\left(u-9\right)$

$u^2-5u-36=\left(u+4\right)\left(u-9\right)=0$

Using the Zero factor Theorem: if ab = 0 then a = 0 or b = 0 (or both a = 0 and b = 0)

The solutions to the quadratic equation are:

$\:u=-4,\:u=9$

Substitute back
u=x^2 , solve for x

x^4-5x^2-36=(u-9)(u+4)=(x^2-9)(x^2+4)

Apply the difference of squares formula

x^4-5x^2-36=(x^2-9)(x^2+4)=(x-3)(x+3)(x^2+4)

(x-3)(x+3)(x^2+4)=0

Using the Zero factor Theorem: if ab = 0 then a = 0 or b = 0 (or both a = 0 and b = 0)

The solutions are:

$\:x=3,\:x=-3,\:x=2i,\:x=-2i$

Because two of the solutions are complex roots the only x-intercepts are
$x=3,\:x=-3$