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Question
If sin θ = `4/5` then find cos θ.
Sum
Solution
sin2θ + cos2θ = 1
⇒ `(4/5)^2 + cos^2 θ = 1 ...( sin θ = 4/5)`
⇒ `16/25` + cos2θ = 1
⇒ cos2θ = `1 - 16/25`
⇒ cos2θ = `(25 - 16)/25`
⇒ cos2θ = `9/25`
⇒ cos θ = `sqrt (9/25)`
⇒ cos θ = `3/5`
Thus, the value of cos θ is `3/5`.
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Trigonometric Ratios of 30° and 60° Angles
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