b1+b1q^2=20

b1q^4-b1=15

b1=15/(q^4-1)

15(1+q^2)/(q^4-1)=20

15=20q^2-20

20q^2=35

q=sqrt(7/4)

q^4=49/16

b1=80/11

b10=80/11*(49/16)^2*sqrt(7/4)=12005/176sqrt(7/4)

2-z задача

b1(2^8-1)/1=765

b1=765/255=3

b8=3*2^7=384

b1(32/243-1)/-1/3=211

b1=81

b5=16