Assume CAPM holds. Given that portfolio beta $$beta subscript P$=0\mathrm{.}5$ beta subscript A plus $0\mathrm{.}5$ beta subscript B$,$ show$/$prove that E$($Rp$)=0\mathrm{.}5$E$($RA$)+0\mathrm{.}5$E$($RB$).$