4 Quick Math Questions for Grade 11

[Raza]

1)~How~would~I~put~this~into~exact~ratio?

Cos\frac{5pi}{6}



2)~How~would~factor~this~expression?

Cosx-2SinxCosx



This~is~not~a~quick~question~but~I~think~it~is~tou gh.~I~got~1~out~of~9~in~it. :grumpy:
3)~How~would~I~solve~this~equation?

6Cosx^2x+Sinx-4=0~for~ 0^0<x<360^0



4)~How~many~solutions~does~the~equation~\sqrt 2cos(2x)=1~for~0<x<2pi~have?

Any~help~will~be~appreciated!

THANK~YOU :smile:

[whozum]

You give it a try first.

[Raza]

I know the answer to the 1st question is -0.86602540378443864676372317075294 but how do I convert this into a fraction? My guess the answer has to be Cos(\frac{5pi}{1})

As for the 2nd question, my teacher gave me a zero on this question and I don't know what I did wrong.
(Cosx-Sinx)(Cosx+Sinx)

For the 3rd question,
6Cosx^2x+Sinx-4=0~for~ 0^0~<0<360^0 factors into (3Cosx-2)(3Cosx+2)
| |
| |
| |
| |-------------->2Cosx+2=0
| |-------------->Cosx=-\frac{2}{2}
| |-------------->Cosx=-1
|
|
3Cosx-2=0
Cosx=\frac{2}{3}

[whozum]

-0.86602540378443864676372317075294 reduces to a very common ratio in trig. Do you have a unit circle handy? Or some trig tables? Maybe drawing out a 30-60-90 triangle with hypotenuse 1, short side 0.5. Solve for the long side.

[Raza]

Thank you for help me with question 1, it's -\frac{\sqrt3}{2}

[James R]

The factorisation question...

\cos x - 2 \sin x \cos x

You answered:

(\cos x-\sin x)(\cos x+ \sin x)

But if we expand that out we get:

\cos^2 x - \sin^2 x

which is obviously not the same as the original expression. The best you can do for the original expression is:

\cos x - 2 \sin x \cos x = \cos x(1 - 2 \sin x)

[whozum]

If you are allowed double angles, you can get
\cos x - \sin 2x which is a bit simpler.

[Raza]

I understand how to do 1 and 2 (Thank You) but how would you do the 4th one? That is the one where I got 1 out 9 on the unit test.

[whozum]

\sqrt 2 \cos 2x = 1 for x.

Divide both sides by \sqrt 2 .

Then for simplicity, say u = 2x.

\cos u = \frac{1}{\sqrt{2}} = \frac{\sqrt 2}{2}

Solving that gives you u. Then use the u = 2x equation above to find x.

[Raza]

I have tried to figure out question 3 myself. Could someone please check my work?

6Cos^2x+Sinx-4=0~for~ 0^0

6Cos^2x+Sinx=4

6(1-Sin^2x)+Sinx=4

1-Sin^2x+Sinx=\frac{4}{6} *Rearrange it as Sin2x+1-Sinx and factor it

(Sinx+1)(Sinx-1)=\frac{4}{6}

For (Sinx-1)
Sinx-1=\frac{4}{6}
Sinx=-\frac{5}{3}

For (Sinx+1)
Sinx+1=\frac{4}{6}
Sinx=\frac{5}{3}

[whozum]

Between lines 3 and 4, you divide both sides by 6, you must also divide the single (sinx) term by 6.

Also, between lines 4 and 5 you factor 1 - sin^2x + sinx incorrectly. You seem to have trouble with factoring, try reviewing some. When you expand the binomials you wrote you get [itex] sin^2x - 1 [/tex] which isnt equal to the original expression.

Also note, sinx is always bounded by -1 and 1. Your two answers lie outside these bounds.

[Ouabache]

I have tried to figure out question 3 myself. Could someone please check my work?

6Cos^2x+Sinx-4=0~for~ 0^0

6Cos^2x+Sinx=4

6(1-Sin^2x)+Sinx=4
1-Sin^2x+Sinx=\frac{4}{6}

I see a problem. On the left side you neglected to divide \frac{Sinx}{6}
then you would have: 1-Sin^2x+\frac{Sinx}{6}=\frac{4}{6} which isn't too easy to work with.

instead, how about multiplying through and factoring?
6(1-Sin^2x)+Sinx=4
6-6Sin^2x+Sinx-4=0
-6Sin^2x+Sinx+2=0 .. you get the idea? :rolleyes:

[edition]: I see whozum spotted the same trouble.. If you factor as I suggest, you will get two solutions that do lie on the unit circle (bounded by +1 and -1)