prove that sin21 cos9-cos84 cos6=1/4
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Step-by-step explanation:
sin21.cos9-cos84.cos6
=>sin21.cos9-cos(90-6).cos6
=>sin21.cos9-sin6.cos6
=>1/2{2sin21.cos9-2sin6.cos6}
use formula
2sinA.cosB=sin(A+B)+sin(A-B)
=>1/2{sin30+sin12-sin12-sin0}
=>1/2{sin30}
=>1/2 x 1/2=1/4
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