# Thread: I can't think tonight or something

1. sin(arcsin(8x))
thats 8x

2. Originally Posted by ElijahWyatt
arccos(2x)=arcsec(2x)
arcsec(x) = arccos(1/x) so
arccos(2x)=arccos(1/2x)
cos both sides.
2x=1/2x
4x^2=1
x=1/4
x = multiples of pi/2

3. lol i did that wrong. its 1 or -1

4. actually 1/2 or -1/2, you know what i mean

5. why? What was wrong with my "proof"?
and i'm right btw
Meaning my answer of x=1/4 is right.

but ya, -1/4 too, because square root.

6. you actually did it right looking at it, but you didnt actually take the square root

7. Ok I got all these now except for one, God was right the whole time lol.

The only one I can't figure out is:

Let f(x)=3x^5+x^3+1
f^-1(5)

I can't think of an easy way to do this besides switching the x and y, then right off the bat plugging in 5, which doesn't work.

8. Yeah I go in giant circles:

(1) 3x^5 - x^3 = y + 1
(2) x^3(x-1) = y + 1
(3) x-1 = (y+1)/x^3
(4) x = (y+1)/x^3 + 1
(5) x/(y+1) = 1/x^3 + 1/(y+1)
(6) x/(y+1) - 1/(y+1) = 1/x^3
(7) x^4/(y+1) - x^3/(y+1) = 1
(8) (x^4-x^3)/(y+1) = 1
(9) x^3(x-1) = y+1 (step 2)

9. you won't be able to find an explicit formula for the inverse of a quintic. well, maybe you CAN, but it's pretty advanced stuff. i think. so just look for the value of x that will make f = 5.

10. Yeah I emailed by professor and that is exactly what he said, "just plug in common numbers"...

ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!

11. I feel like I am being trolled.

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