Question

Помогите решить 555 (б)

Answer 1

a(n)=a₁+d(n-1)
1. а₂=а₁+d(2-1)= a₁+d
    a₂₃=a₁+d(23-1)= a₁+22d
a₂+a₂₃= a₁+d+a₁+22d = 2a₁+23d
2. a₁₂=a₁+d(12-1) = a₁+11d
    a13= a₁+d(13-1) = a₁+12d
a₁₂+a₁₃= a₁+11d+a₁+12d= 2a₁+23d
3. 2a₁+23 = 2a₁+23d