b2+b3=b2(1+q)=60
b4-b2=b2(q^2-1)=180
(b4-b2)/(b2+b3)=q-1=3
q=4
b2=12
b4=192
b5=768
b2=b1*q; b3=b1*q^2; b4=b1*q^3
b1*q+ b1*q^2=60; b1*q^3- b1*q=180
b1=60/(q+q^2); b1=180/(q^3-q)
60/(q+q^2)= 180/(q^3-q)
откуда q=4
b1=60/(4+16)=3
b5=b1*q^4=3*256=768
