Respuesta :

cos (theta) = √21/5

Step-by-step explanation:

sin (theta) = 2/5

(sin (theta))^2 = 4/25

4/25 + (cos (theta))^2 = 1

(cos (theta))^2 = 1 - 4/25 = 21/25

cos (theta) = √21/5

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