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S₅ = ar⁵⁻¹ = ar⁴
S₅/S₃ = ar⁴/ar²
80/20 = r²
4 = r²
2 = r
S₃ = ar²
20 = a.4
20/4 = a
5 = a
S₈ = ar⁸⁻¹ = ar⁷
= 5.r⁷
= 5. 2⁷
= 640