2022
Mathematics
JAMB
If sin θ = - 3/5 and θ lies in the third quadrant, find cos θ
A.
4/5
B.
-1 1/4
C.
5/4
D.
- 4/5
Correct Answer: - 4/5
Explanation
Where sin θ = opp/hyp → −3/5 opp = -3, hyp = 5 using pythagoras formula hyp2 = adj2 + opp2 adj2 = hyp2 - opp2 adj2 = 52 - 32 → 25 - 9 adj2 = 16 adj = 4 cos θ = adj/hyp → - 4/5 In third quadrant: cos θ is negative → - 45
Where sin θ = opp/hyp → −3/5 opp = -3, hyp = 5 using pythagoras formula hyp2 = adj2 + opp2 adj2 = hyp2 - opp2 adj2 = 52 - 32 → 25 - 9 adj2 = 16 adj = 4 cos θ = adj/hyp → - 4/5 In third quadrant: cos θ is negative → - 45THIS WEEK's
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