Answer:
a₁₀₀ = 1074
Explanation:
the nth term of an arithmetic sequence is
= a₁ + (n - 1)d
where a₁ is the first term and d the common difference
given a₃ = 7 and a₁₂ = 106 , then
a₁ + 2d = 7 → (1)
a₁ + 11d = 106 → (2)
solve the equations simultaneously to find a₁ and d
subtract (1) from (2) term by term to eliminate a₁
(a₁ - a₁) + (11d - 2d) = 106 - 7
0 + 9d = 99
9d = 99 ( divide both sides by 9 )
d = 11
substitute d = 11 into (1) and solve for a₁
a₁ + 2(11) = 7
a₁ + 22 = 7 ( subtract 22 from both sides )
a₁ = - 15
Then
a₁₀₀ = - 15 + (99 × 11) = - 15 + 1089 = 1074