Question

If three vectors a, b, c satisfy a + b + c = 0 and | a | = 3, | b | = 5, | c | = 7, then the angle between a and b is

Answer 1

|a|=3,|b|=4,|c|=5

a+b+c=0

a+b =-c

|a+b| =|-c|

|a+b|2 =|-c|2

|a|2 +|b|2 +2a.b =|c|2

9 + 25 +2a.b=49

a.b=3*5

9+25+2ab cos theta=49

15=30cos theta

or cos theta=1/2

theata =60

so angle is 60 degrees