5. If the first, second and the last terms of an A.P. are a, b, c. respectively, then tie sum is 15 (a + b)(a +c-2b) 12(b-a) T 2c) th-a) (b + c)(a + b 2 (b-a) (6) RON (a + c)(b +c-2a) 2(b-a) 16. (d) None of these