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_7
d = ---------
2
d=2−a7+a9 a_9 - a_7
a_1 = a_9 - ---------*7
2
a1=a9−2−a7+a9⋅7 d=2−2+8 8 - 2
a_1 = 8 - -----*8
2
a1=(−1)2−2+8⋅8+8