0000000000000000000000000000000000000
b3-b1=42 b1q^2-b1=42 b1(q^2-1)=42
=d²+2d-9d-18-d²+15d-6d+90=2d+72.
при d= -0,12, 2d+72= -0,12·2+72= -0,24+72=71,76.