Question

Let a1,a2,......a10 be an arithmetic sequence. If a1+a3+a5+a7+a9=17 and a2+a4+a6+a8+a10=15 then find a1.

Answer 1

a1+...+a9 = 5a + (0+2+4+6+8)d
a2+...+a10 = 5a + (1+3+5+7+9)d

so,

5a+20d = 17
5a+25d = 15
d = -2/5
a = 5

Answer 2

A6, A7,A8,A9 and A10. A1 is 5, d is 2