How do you find the exact value of $cos (3pi)$ ?

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Answer 1 · 2016-04-16T21:58:26

We have that

$cos3pi=cos(2i+pi)=cos2pi*cospi-sin2pi*sinpi= 1*(-1)-0*0=-1$

Also

$cos(3pi)=cos(pi)=-1$

Footnote

We used the trig fromula

$cos(A+B)=cosA*cosB-sinA*sinB$

How do you find the exact value of cos (3pi)cos (3pi)?