In a triangle ABC, if the three sides are a=5, b=7 and c=3, what is angle B?
Answer & explanation
Correct answer: option 1
By using cosine rule :-
cos B = \(\frac{ 5² + 3² - 7²}{ 2 × 5 × 3}\)
= \(\frac{ 25 + 9 - 49}{ 30}\)
= \(\frac{ - 15}{ 30}\)
= \(\frac{ - 1}{ 2}\)
cosB = cos 120º
So, B = 120º