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Question Number 47639 by azharkhan250963@gmail.com last updated on 12/Nov/18

derivation or proof or full explanation  of R=(R_1 ^2 +R_2 ^2 +2R_1 .R_2 cosθ)^(1/2)

derivationorprooforfullexplanationofR=(R12+R22+2R1.R2cosθ)1/2

Answered by tanmay.chaudhury50@gmail.com last updated on 12/Nov/18

R^→ =R_1 ^→ +R_2 ^→   R^→ .R^→ =(R_1 ^→ +R_2 ^→ ).(R_1 ^→ +R_2 ^→ ).....(1)  [now A^→ .B^→ =∣A^→ ∣∣B^→ ∣cosα=abcosα  ∣A^→ ∣=a    ∣B^→ ∣=b     where α is angle between A^→  and B^→   again P^→ .P^→ =∣P^→ ∣∣P^→ ∣cos0^o =∣P^→ ∣^2 =p^2    ]    so from 1 we get  R^→ .R^→ =R_1 ^→ .R_1 ^→  +2R_1 ^→ .R_2 ^→ +R_2 ^→ .R_2 ^→   R^2 =R_1 ^2 +R_2 ^2 +2R_1 R_2 cosθ  R=(√(R_1 ^2 +R_2 ^2 +2R_1 R_2 cosθ))    where   ∣R^→ ∣=R    ∣R_1 ^→ ∣=R_1     ∣R_2 ^→ ∣=R_2

R=R1+R2R.R=(R1+R2).(R1+R2).....(1)[nowA.B=∣A∣∣Bcosα=abcosαA∣=aB∣=bwhereαisanglebetweenAandBagainP.P=∣P∣∣Pcos0o=∣P2=p2]sofrom1wegetR.R=R1.R1+2R1.R2+R2.R2R2=R12+R22+2R1R2cosθR=R12+R22+2R1R2cosθwhereR∣=RR1∣=R1R2∣=R2

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