12. 1+ 2+ 3+ 4+ … + 20 = …. A. 201 B. 210 C. 220 D. 420​

Jawab:

Penjelasan dengan langkah-langkah:

a = 1

b = 2 – 1 = 1

S20 = (20/2)(2(1) + (20 – 1)1)

S20 = 10 (2 + 19)

S20 = 10 (21)

S20 = 210 (B)