Grade 11Trigonometry
Prove that cos π/5.cos 2π/5=1/4.And cos π/5 -cos 2π/5=1/2
Prove that cos π/5.cos 2π/5=1/4.
And cos π/5 -cos 2π/5=1/2
1 Answer
anmol
quite easy,
1/2(2cospi/5cos2pi/5)-1/4
2cosAcosB=cos(A+B)+cos(A-B)
cos3pi/2+cospi/5-1/2
cos(pi-2pi/5)+cospi/5=1/2
cospi/5-cos2pi/5=1/2,
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