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# Solving cos(θ)=1 and cos(θ)=-1

Sal solves the equations cos(θ)=1 and cos(θ)=-1 using the graph of y=cos(θ). Created by Sal Khan and Monterey Institute for Technology and Education.

Video transcript

In the graph below, for what values of theta does cosine of theta equal one and for what values of theta does cosine of theta equal negative one? And they have very nicely graphed it for us. The horizontal axis is the theta axis and the vertical axis is the y axis and so this is the graph of y is equal to cosine of theta. And it makes sense with the unit circle definition and I'll just make sure we're comfortable with that because the unit circle definition so let me draw ourselves a unit circle and I'm going to draw it very roughly just so we get the general idea of what's going on here. When theta is equal to zero Where this point right over here on the unit circle. Well what's the x coordinate of that point? Well it's one and you see when theta is equal to zero on this graph, cosine of theta is equal to one when theta is equal to pi over 2 we're at this point on the unit circle and the x coordinate is what