b1+b4=7/16
b3-b2+b1=7/8
b1+b1q³=7/16
b1q²-b1q+b1=7/8
b1( 1+q³)=7/16
b1(q²-q+1)=7/8
impartim prima ecuatie la a doua si coinsideram cunoscuta formula
1+q³=(1+q)(1-q+q²)
avem atunci
1+q=(7/16):(7/8)
1+q=1/2
q=1/2-1=
q=-1/2
din b1(1+q³)=7/16
avem b1( 1-1/8)=7/16
adica b1* (7/8)=7/16
de unde
b1=(7/16) :7/8=7/16*8/7=1/2
deci b1=1/2
q=-1/2, cerinta
