APPROACH PLEASE

a.the sum of products of ten numbers (both positive &negative; signs)1,2,3,...5 taking two at a time is ?
B.If a ,b ,c are real numbers ,then minimum value of ab+bc+ca?C. If |x| -|y| =13 then which of following can't be value of x-y? options: -19 / -9/-17/none of these