Homework Help: math

Posted by raja on Saturday, January 5, 2013 at 4:04am.

if vectors a+b+c=0 and modulus of the vectors a, b and c are 3 ,5 and 7 respectively. find the angle between the vectors a and b.

math - Reiny, Saturday, January 5, 2013 at 8:07am
let the angle be Ų
by the cosine law

7^2 = 3^2 + 5^2 -2(3)(5)cosŲ
30cosŲ = 9+25-49
cosŲ = -15/30 = -1/2
Ų = 150°

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