an = a1 + d(n-1)
a2 = a1+d
a3 = a1+2d
a4 = a1+3d

נציב במשוואות
1) a1*(a1+d) = 4
2) (a1+2d)*(a1+3d)=70

1) a1^2 + a1d = 4
2) a1^2 + 3a1d + 2a1d + 6d^2 = 70

ממשוואה אחת למשל
d = (4-a1^2) / a1

נציב בשניה

a1^2 + 5a1*(4-a1^2)/a1 + 6 * (4-a1^2)^2/a1^2 = 70 /*a1^2
a1^4 + 5a1^2*(4-a1^2) + 6*(16 - 8a1^2 + a1^4) = 70a1^2
a1^4 + 20a1^2 - 5a1^4 + 96 - 48a1^2 + 6a1^4 = 70a1^2
2a1^4 - 98a1^2 + 96 = 0
a1^4 - 49a1^2 + 48 = 0
a1^2 = t
t^2 - 49t + 48 = 0
t^2 - 48t - t + 48 = 0
(t-1)(t-48) = 0
t = 1 , 48

a1^2 = 1
a1 = 1, -1
או
a1 = sqrt(48) , -sqrt(48)