banyak persegi pada pola ke-20 adalah?

Banyak persegi =(1+20).20/2
=21.10
=210

u1 = 1
u2 = 3
u3 = 6
u4 = 10

u5 = 10 + 5 = 15
u6 = 15 + 6 = 21
u7 = 21 + 7 = 28
u8 = 28 + 8 = 36
u9 = 36 + 9 = 45
u10 = 45 + 10 = 55
u11 = 55 + 11 = 66
u12 = 66 + 12 = 78
u13 = 78 + 13 = 91
u14 = 91 + 14 = 105
u15 = 105 + 15 = 120
u16 = 120 + 16 = 136
u17 = 136 + 17 = 153
u18 = 153 + 18 = 171
u19 = 171 + 19 = 190
u20 = 190 + 20 = 210 
jawabannya =  c.210