1. 2; 3; 4; 5; 6; 7. N 1; 2; 3; 4; 5; 6; 7 ........n 0; 1; 3; 6; 10; 15; 21 ........( )

1. 2; 3; 4; 5; 6; 7. N 1; 2; 3; 4; 5; 6; 7 ........n 0; 1; 3; 6; 10; 15; 21 ........( )

Mark the first number as A1 and the second as A2
because
a2 - a1 = 1
a3 - a2 = 2
a4 - a3 = 3
.
an - an-1 = n-1
Add the left and right sides of the above formula to get
an - a1 = 1+2+3+.+(n-1)
therefore
an = (n-1)n/2 +a1 = (n-1)n/2

Elden Ring Premiere

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