(a) : In such type of problem if sum of the squares of number is known and we needed product of numbers taken two at a time or needed range of the product of numbers taken two at a time. We start square of the sum of the numbers like
(a + b + c)2 = a2 + b2 + c2 + 2(ab + bc + ca)
=> 2(ab + bc + ca) = (a + b + c)2 – (a2 + b2 + c2)