solve for x

**tbenne3** · March 31st 2010, 02:41 PM

cos2x=0

x=pi/4 but how?

**skeeter** · March 31st 2010, 02:55 PM

Originally Posted by tbenne3

cos2x=0

x=pi/4 but how?

because $\cos\left(2 \cdot \frac{\pi}{4}\right) = \cos\left(\frac{\pi}{2}\right) = 0<br />$

also note that there are more solutions ... $x = \frac{3\pi}{4}$ for instance.

**pickslides** · March 31st 2010, 02:56 PM

Originally Posted by tbenne3

cos2x=0

x=pi/4 but how?

make $2x = \theta$

so $\cos \theta = 0 \implies \theta = \frac{\pi}{2} \implies 2x = \frac{\pi}{2} \implies x = \frac{\pi}{4}$

**pitoten** · March 31st 2010, 03:43 PM

Remember that we are talking about radians and not degrees! Review the unit circle. You will see that wherever the circle crosses the y axis, the cosine of that value is always zero. If you type in [pi]/4 into your calculator and it is set in degrees, you will be very confused.

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