Zhan,
I agree with Douglas.
cos(11pi/12)=cos (8pi/12+3pi/12)=cos(8pi/12)*cos3pi/12)-sin(8pi/12)*sin(3pi/12)
cos(11pi/12)=(-1/2)*.707-(3(1/2)/2)*.707
.707=2(1/2)/2

Zhan Y.
asked 11/25/20can you please show me how to do this
Zhan,
I agree with Douglas.
cos(11pi/12)=cos (8pi/12+3pi/12)=cos(8pi/12)*cos3pi/12)-sin(8pi/12)*sin(3pi/12)
cos(11pi/12)=(-1/2)*.707-(3(1/2)/2)*.707
.707=2(1/2)/2
Douglas B. answered 11/25/20
Algebra 2 Expertise
Think about what you know. There are only a few angles of which we know the exact values of cosine. These are the angles on the unit circle:
0,pi/6,pi/4,pi/3,pi/2,2pi/3,3pi/4,5pi/6,pi.
These are all the exact angles up to pi.
Now, you need to figure out how to write 11pi/12 in terms of the above angles (as a sum).
Once you do this, you can use an angle-sum identity to determine cos(11pi/12).
Zhan Y.
thank you very much Douglas11/25/20
Get a free answer to a quick problem.
Most questions answered within 4 hours.
Choose an expert and meet online. No packages or subscriptions, pay only for the time you need.
Dayv O.
11/25/20