Chapter 7 - Section 7.2 - Arithmetic Sequences and Series - 7.2 Exercises - Page 645: 25

$a_1=5$

Sum formula: $$S_n=\frac n2(a_1+a_n)$$ Use $n=16$: $$S_{16}=\frac {16}2(a_1+a_{16})$$ Substitute $S_{16}=-160$ and $a_{16}=-25$: $$-160=\frac {16}2(a_1+(-25))$$ Solve for $a_1$: $$-160=8(a_1-25)$$ $$a_1-25=-20$$ $$a_1=5$$