In a triangle ABC, if the three sides are a = 5, b = 7 and c = 3, what is angle B?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 120°.
Given:
a = 5, b = 7 and c = 3
Concept used:
cosine rule is used.
Calculations:
Using cosine rule, we get
⇒ cosθ = (a2 + c2 - b2)/2ac
⇒ cosθ = (52 + 32 - 72)/(2 × 5 × 3)
⇒ cosθ = (25 + 9 - 49)/(2 × 5 × 3)
⇒ cosθ = (-15)/(30)
⇒ cosθ = -1/2
⇒ θ = 120°