Question

if vector A+B+ VECTOR C and the angle between A and b is 120 what is the magnitude of c

Answer 1

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We have,

a+b=c……………………(1)

Also,

|a|+|b|=|c| …………(2)

Square the equation, (1), we get,

(a+b).(a+b)=c.c

Implies,

|a|2+|b|2|+2(a.b)=|c|2 ……….(3)

From (2), we can get,

|a|2+|b|2|+2|a||b|=|c|2 ………(4)

Since the RHS’s of (3) and (4) are same, we can equate them, we get,

|a|2+|b|2|+2(a.b)=|a|2+|b|2|+2|a||b|

Implies,

a.b=|a||b|

Dot Product of 2 vectors is the product of absolute values of the vectors with the cosine of angle between them. So,

|a||b|cosx=|a||b|

Here, xx is the angle between a and b

So,

cosx=1

Take cosine inverse or arccosarccos on both sides, we get,

x=cos−1(1)

Implies,

▶x=0

So, the angle between the 2 given vectors is 0, which means they’re parallel.