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Mathematics 12 Online
OpenStudy (anonymous):

Use a sum or difference formula to find the exact value of the following: sin(5π/7)cos(π/21)-cos(5π/7)sin(π/21)

OpenStudy (cwrw238):

use the identity sin (a - b) = sin a cos b - cos a sin b

OpenStudy (cwrw238):

so your expression becomes sin (5pi/7 + pi/21)

OpenStudy (cwrw238):

= sin 16pi/21

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