Posted by **Crystal** on Monday, February 23, 2009 at 7:58pm.

how do you solve for x?

2cos^24x-1=0

- math (trig) -
**myschic**, Monday, February 23, 2009 at 8:20pm
First find the cos of 24

multiply this by 2

add 1 to the other side

Finally, divide that answer with 1

and... u have the answer!!! whatever that is...

- math (trig) -
**Reiny**, Monday, February 23, 2009 at 8:31pm
I am reading your question as

2cos^2 (4x) - 1 = 0

so cos^2 (4x) = 1/2

cos 4x = ±1/√2

so 4x must be an angle in each of the 4 quadrants.

but I know cos 45º = 1/√2

so 4x = 45 or 4x = 135 or 4x = 225 or 4x = 315

then

x = 11.25 or x = 33.75 or x=56.25 or x=78.75

the period of cos 4x = 90º

so by adding/subtracting multiples of 90 to any of the above answers will produce other answers

e.g. 33.75 + 90 = 123.75 would be another possible answer

proof:

LS

= 2 cos^2 (4*123.75) - 1

= 2cos^2 (495) - 1

= 2(1/2) - 1

= 0 = RS

- math (trig) -
**mysterychicken**, Monday, February 23, 2009 at 8:35pm
just follow my way, its much simpler

- math (trig) -
**Reiny**, Monday, February 23, 2009 at 8:53pm
MC, except you don't get the right answer, lol

- math (trig) -
**mysterychicken**, Tuesday, February 24, 2009 at 6:35pm
prove it Reiny

:)

my way always works

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