Answer:
(b) cos(3π/10)
Step-by-step explanation:
The given expression matches the trig identity form for the cosine of the difference of two angles:
cos(α-β) = cos(α)cos(β) +sin(α)sin(β)
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To match the given expression exactly, we can choose ...
α = π/2
β = π/5
Then the difference is ...
α -β = π/2 -π/5 = (5/10)π -(2/10)π = 3π/10
The given expression can be shortened to ...
cos(3π/10)
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<em>Additional comment</em>
Sometimes it can be difficult to remember when the signs in trig identities match, and when they differ. The fact that cosines of smaller angles have larger values can be a peg on which to hang that hat.