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_9 - a_1
d = ---------
8
d=8−a1+a9 a_9 - a_1
a_1 = a_9 - ---------*7
8
a1=a9−8−a1+a9⋅7 d=8−7+8 8 - 7
a_1 = 8 - -----*8
8
a1=(−1)8−7+8⋅8+8