Let `N` be the set of all natural numbers and let `R` be a relation on `NxxN` , defined by `(a ,\ b)R\ (c ,\ d) <=>a d=b c` for all `(a ,\ b),\ (c ,\ d) in NxxN` . Show that `R` is an equivalence relation on `NxxN` . Also, find the equivalence class [(2,6)].