" Life is not a problem to be solved but a reality to be experienced! "
© Copyright 2013 - 2024 KUDO.TIPS - All rights reserved.
r= -1/2
an=a1+(n-1)*r
a10=16 -9/2=16-4,5=11,5
a20=16-19/2=16 -9,5=6,5
n=11
S(10-20)=(a10+a20)*11/2
S(10-20)=(11,5+6,5)*11/2=18*11/2=9*11=99
S(10-20)=99