1) a4 = 1.8, a7 = 0.6
a1 - ?, d - ?
d = (a7 - a4)/(7 - 4) = -0.4
a1 = a4 - 3d = 3
2)b2 = -2, b4 = -6
b4 = b2 * q^2
q^2 = 3
1 случай:
q = sqrt(3)
b7 = b2*q^5
b7 = -2 * 9sqrt(3) = -18sqrt(3)
2 случай:
q = -sqrt(3)
b7 = b2*q^5
b7 = -2 * (-9sqrt(3)) = 18sqrt(3)