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- Thread starter kbr1804
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- Mar 1, 2012

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use the sum/difference identity $\cos(a \pm b) = \cos{a}\cos{b} \mp \sin{a}\sin{b}$

it should be equal to $3\cos(2x)$, not $3\cos{x}$

it should be equal to $3\cos(2x)$, not $3\cos{x}$

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- Feb 21, 2015

- 113

How can i prove that 6cos(x+45) cos(x-45) is equal to 3cosx?

use the sum/difference identity $\cos(a \pm b) = \cos{a}\cos{b} \mp \sin{a}\sin{b}$

it should be equal to $3\cos(2x)$, not $3\cos{x}$

Three graphs; shouldn't 2 of them have the same graph?yeah i think i got it lol thanks alotand yeah it was supposed to equal to 3cos2x that was a typo

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- Mar 1, 2012

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- Jan 30, 2012

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- Mar 1, 2012

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One can change to degree mode with the "wrench" button menujonah, in Desmos one should write $\pi/4$ instead of 45.

- Feb 21, 2015

- 113

jonah, in Desmos one should write $\pi/4$ instead of 45.

Well aware of that.One can change to degree mode with the "wrench" button menu

Force of habit.

In the absence of the degree symbol (°), assumed that 45 was in radians.

Didn't occur to me to check it in degree mode.

Usually do it by multiplying the degree measure by $\frac{\pi}{180}$ if the expression just gives it once. Otherwise, I usually do it by skeeter's suggestion (which I'm also quite aware of).

I've often wondered what kind of platform this type of Desmos "quoting" is ever since I saw one of Klaas van Aarsen's post which used the same method. It isn't just a link to Desmos as I found out when I hit the reply tab on my phone. Is it that the TikZ thingamajig I've been seeing a lot lately on this site? I think I remember copying that stuff in another math site but was surprised that it didn't work there.... and on Desmos ...

- Jan 30, 2018

- 881

You CAN'T- it's not true! For example if x= 45 degrees this becomes 6 cos(90)cos(0)= 6(0)(1)= 0 but 3 cos(45)= 3sqrt(2)/2.How can i prove that 6cos(x+45) cos(x-45) is equal to 3cosx?

- Mar 1, 2012

- 1,017

post #2 …You CAN'T- it's not true! For example if x= 45 degrees this becomes 6 cos(90)cos(0)= 6(0)(1)= 0 but 3 cos(45)= 3sqrt(2)/2.

it should be equal to $3\cos(2x)$, not $3\cos{x}$