Exact value of cos^273^o + cos^247^o + (cos73^o . cos47^o ) is

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3.Trigonometrical Ratios, Functions and Identities

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The exact value of $cos^273^o + cos^247^o + (cos73^o . cos47^o )$ is

A

$1/4$

B

$1/2$

C

$3/4$

D

$1$

Solution

$\frac{{1 + \cos {{146}^o }}}{2}$ $+$$\frac{{1 + \cos {{94}^o }}}{2}$ $+$ $\frac{{\cos {{120}^o } + \cos {{26}^o }}}{2}$

$= 1 +$ $\left[ {\frac{{\cos {{146}^o }\, + \cos {{94}^o }}}{2}} \right]\,\, – \,\,\frac{1}{4}\, + \,\frac{{\cos {{26}^o }}}{2}$

$= 3/4$

Standard 11

Mathematics

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