Factorise the following expressions. (i) `a^2+8x+16` (ii) `p^2-10p+25` (iii) `25m^2+30m+9` (iv) `49y^2+84yz+36z^2` (v) `4x^2-8x+4` (vi) `121b^2-88bc+16c^2` (vii) `(l+m)^2-4lm` (Hint : Expand `(l+m)^2}` first (viii) `a^4+2a^2 b^2 +b^4`