Question

a=-20, a16=-80 find d and s16

Answer 1

a=-20
n=16
l =-80
a + (n-1)d=l
-20 +(16-1)d=-80
15d=-60
d=-4

S16=16/2 (-20-80)
=8*-100
=-800

Answer 2

a= - 20.
$a \binom{}{16} = - 80 = a + 15d$
$- 20 + 15d = - 80 \\ 15d = - 60 \\ d = - \frac{60}{15} \\ d = - 4$


Now s(n) = n/2(a+l). [l:last term]
$s \binom{}{16} = \frac{16}{2} (( - 20) + a \binom{}{16} )$
$= 8( - 20 + (- 80)) \\ = 8( - 100) \\ = - 800$