" Life is not a problem to be solved but a reality to be experienced! "
© Copyright 2013 - 2025 KUDO.TIPS - All rights reserved.
a11=a1+10r
a21=a1+20r
a21-a11 =20r-10r
10r=20-30 /:10
r=-1
a20=a21-r
a20=20-(-1)=20+1 =21
a40=a20+20r
a40=21-20*1
a40=1
a20=21