A = 12 + 22 + 32 +.....+202
A = 1 + (1+1).2 + (2+1).3+....+ (19+1).20
A = 1 + 1.2 + 2 + 2.3 + 3 +....+ 19.20 + 20
A = (1+2+3+...+20) + ( 1.2+2.3+...+19.20)
A =(20+1).{ (20-1):1+1):2 + \(\dfrac{1}{3}\).( 1.2.3+2.3.3+....+19.20.3
A =210 + \(\dfrac{1}{3}\).{ 1.2.3 +2.3.(4-1)+....+19.20.(21-18)}
A = 210 + \(\dfrac{1}{3}\) . { 1.2.3 - 1.2.3 + 2.3.4+.....-19.20.18+ 19.20.21}
A = 210 + \(\dfrac{1}{3}\). 19.20.21
A = 210 + 2660
A = 2870