Find a1 for the geometric series r=-1 and s11=7

An infinite  geometric series   is the sum of an infinite  geometric sequence . this series would have no last term. the general form of the infinite geometric series is  a1+a1r+a1r2+a1r3++a1r+a1r2+a1r3+   , where  a1a1   is the first term and  rr   is the common ratio.

1. 0.3 •4 is the same as 4 groups of 0.32. 7•0.8 is the same as 7 groups of 0.83. it increases by 0.5 every time, so 0.5x4 it increases by 0.9 every time, so 0.9x5. 0.9•12=10.86. 0.47•5=2.35i can finish the next ones in a separate post