Q. The value of cos²73°+cos²47°+cos73°cos47° is
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=1/2[1+cos(94)+1+cos(146)+cos(120)+cos(26)]
=1/2[2+cos(94)+cos(146)-(1/2)+cos(26)]
=1/2[3/2+{cos(94)+cos(26)}+cos(146)]
=1/2[3/2+2cos(120)cos(34)+cos(146)]
=1/2[3/2+2(1/2)cos(34)+cos(146)]
=1/2[3/2+cos(34)+cos(146)]
=1/2[3/2+2cos(90)cos(56)]
=1/2[3/2+2(0).cos(56)]
=1/2[3/2+0]
=3/4 Ans.
=1/2[2+cos(94)+cos(146)-(1/2)+cos(26)]
=1/2[3/2+{cos(94)+cos(26)}+cos(146)]
=1/2[3/2+2cos(120)cos(34)+cos(146)]
=1/2[3/2+2(1/2)cos(34)+cos(146)]
=1/2[3/2+cos(34)+cos(146)]
=1/2[3/2+2cos(90)cos(56)]
=1/2[3/2+2(0).cos(56)]
=1/2[3/2+0]
=3/4 Ans.
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