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_2 - a_1
d = ---------
1
d=1−a1+a2 a_2 - a_1
a_1 = a_2 - ---------*0
1
a1=a2−1−a1+a2⋅0 d=13+4 4 + 3
a_1 = 4 - -----*1
1
a1=(−1)13+4⋅1+4