Question

prove that cos20°cos40°-sin5°sin25°=√3+1\4​

Answer 1

=1/2[cos60°+cos20°-cos20°+cos30°]

=1/2[cos60°+cos30°]

=1/2(1/2+√3/2)

taking lcm,

=(√3+1)/4 Answer:

cos 20°cos40°-L.H.S=sin5°sin25°=1/2[2cos20°cos40°-2sin25°sin5°]

=1/2[cos(40°+20°)+cos(40°-20°)-{cos(25°-5°)-cos(25°+5°)}]

............................................. proved.....................................

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