35+31+27+23+.... jika banayak suku 41 tolong di jawab

a = 35
U₂ = a + b
31 = 35 + b
b = 31 - 35
b = -4

Sₙ = n/2 (2a + (n - 1)b)
S₄₁ = 41/2 (2(35) + (41 - 1)(-4))
S₄₁ = 41/2 (70 + (40)(-4))
S₄₁ = 41/2 (70 - 160)
S₄₁ = 41/2 (-90)
S₄₁ = -1845