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Stiind ca a4+a18=12 aflati a5+a17=?

Răspuns :

a₄+a₁₈ = 12

a₄ = a₁+3r

a₁₈=a₁+17r =>a₄+a₁₈ = 2a₁+20r = 12

a₅=a₁+4r

a₁₇ = a₁+16r => a₅+a₁₇ = 2a₁+20r = 12

a₅+a₁₇ = 12