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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Additional comment
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.
