S₁₄=518
a₁=50
n=14
r=?
S₁₄=(a₁+a₁+13r)*14/2
518=(2a₁+13r)*14/2
1036=28a₁+182r
1036=28*50+182r
182r=1036-1400
182r=-346
r=-2
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S₁₄=518
a₁=50
n=14
r=?
S₁₄=(a₁+a₁+13r)*14/2
518=(2a₁+13r)*14/2
1036=28a₁+182r
1036=28*50+182r
182r=1036-1400
182r=-346
r=-2