Find `R_(AB)` in the circuit shown in figure.

Explanation

From triangle OAB: `vec r+ Delta vec r =vec r_(2) rArr vec r_(2)- vec r_(1) = Delta vec r`
Hence, `|vec S|=|Delta vec r|=|vec r_(2)- vec r_(1)|=sqrt(r_(1)^(2)+r_(2)^(2)-2r_(2)r_(2) cos theta)`