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Motion In A Plane

Question
CBSEENPH11018073

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Solution

The magnitude of the resultant of two vectors space straight P with rightwards arrow on top space and space straight Q with rightwards arrow on top acting at angle straight theta is given by, 
                      space space straight R equals square root of straight P squared plus straight Q squared plus 2 PQcosθ end root
Here,       P = A + B,        Q = A - B
R e s u l t a n t space comma space straight R space equals space square root of 3 straight A squared plus straight B squared end root

therefore space P Q space equals space straight A squared space minus space straight B squared

P u t t i n g space t h e space v a l u e s space o f space straight P comma space straight Q comma space straight R space a n d space P Q space i n space a b o v e space e q u a t i o n comma space

square root of 3 straight A squared plus straight B squared end root space equals space square root of left parenthesis straight A plus straight B right parenthesis squared plus left parenthesis straight A minus straight B right parenthesis squared plus 2 left parenthesis straight A squared minus straight B squared right parenthesis space c o s theta end root

T a k i n g space s q u a r e space o n space b o t h space s i d e s comma space

space space space space space space space 3 straight A squared space plus space straight B squared space equals space left parenthesis straight A plus straight B right parenthesis squared space plus space left parenthesis straight A minus straight B right parenthesis squared space plus 2 space left parenthesis straight A squared minus straight B squared right parenthesis c o s theta

rightwards double arrow space space straight A squared space minus space straight B squared space equals 2 space left parenthesis straight A squared minus straight B squared right parenthesis c o s theta

T h a t space i s comma space c o s space straight theta space equals space bevelled 1 half

rightwards double arrow space straight theta space equals space 60 to the power of 0