a_n - a_k
d = ---------
n - k
d=−k+n−ak+an a1=an+d(n−1) (-1 + n)*(a_n - a_k)
a_1 = a_n - --------------------
n - k
a1=an−−k+n(−ak+an)(n−1) a_15 - a_1
d = ----------
14
d=14−a1+a15 a_15 - a_1
a_1 = a_15 - ----------*13
14
a1=a15−14−a1+a15⋅13 86/5 - 58/5
d = -----------
14
d=14−558+586 86 86/5 - 58/5
a_1 = -- - -----------*14
5 14
a1=(−1)14−558+586⋅14+586 a1=558