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U₃+U₆+U₉+U₁₂ = 72
(a+2b)+(a+5b)+(a+8b)+(a+11b) = 72
4a + 26b = 72
Maka,
2a + 13b = 36
Untuk S₁₄
S₁₄ = 14/2 (2a+(14-1)b)
S₁₄ = 7 (2a+13b)
Substitusikan:
S₁₄ = 7 x 36
S₁₄ = 252