QR TRIG question

[tgutberg]

Hey Guys
Can someone please explain to me why

Cos(π/3) = -cos(2π/3)?

Thanks!

Tom
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[z3u2]

try drawing out the 4 quadrant (CAST thing)
you will see that
Cos(π/3) = 1/2 = Cos(60)
cos(2π/3) = -1/2 = Cos(120)

Or you can draw the Cosine graph, you will see the values

 

[Streetwolf]

Hey Guys
Can someone please explain to me why

Cos(π/3) = -cos(2π/3)?

Thanks!

Tom
​

Can also use the rule cos(x-y) = cos(x)cos + sin(x)sin

cos(pi/3) = cos(pi - 2pi/3) = cos(pi)cos(2pi/3) + sin(pi)sin(2pi/3)

Since cos(pi) = -1 and sin(pi) = 0, we're left with cos(pi/3) = -cos(2pi/3)