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Question

If R=(A+B), Show that R2=A2+B2+2 AB cosθ

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Solution

¯¯¯¯R2=¯¯¯¯R.R¯¯¯¯R2=(¯¯¯¯A+¯¯¯¯B).(¯¯¯¯A+¯¯¯¯B)
¯¯¯¯R2=¯¯¯¯A.¯¯¯¯A+¯¯¯¯A.¯¯¯¯B+¯¯¯¯B.¯¯¯¯A+¯¯¯¯B.¯¯¯¯B
(¯¯¯¯A.¯¯¯¯B=¯¯¯¯B.¯¯¯¯A(fordotproduct))
¯¯¯¯R2=¯¯¯¯A.¯¯¯¯Acos(0)+¯¯¯¯A.¯¯¯¯Bcosθ+¯¯¯¯B.¯¯¯¯Acosθ
+¯¯¯¯B.¯¯¯¯Bcos(0)
¯¯¯¯R2=¯¯¯¯A2+¯¯¯¯B2+2¯¯¯¯A.¯¯¯¯Bcosθ
(where¯¯¯¯R2isdotproductofR.Ror¯¯¯¯R2)

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