_{n }= n(a

_{1}+ a

_{n})/2

_{n }is the sum, n is the number of terms, a

_{1 }is the first term, and a

_{n}is the last term.

_{n }= 20(3 + 98)/2

_{n}= 1010

Scott S.

02/16/18

Chrissie B.

02/25/18

Scott S.

_{n }in two different ways.

_{n}= a

_{1}+ (a

_{1}+d) + (a

_{1}+ 2d) + . . . + a

_{n}

_{n}= a

_{n}+ (a

_{n}- d) + (a

_{n}- 2d) + . . . + a

_{1}

_{n}= (a

_{1}+ a

_{n}) + (a

_{1}+ a

_{n}) + . . . + (a

_{1}+ a

_{n}) or 2S

_{n}= n(a

_{1}+ a

_{n})

02/28/18

Scott S.

_{n }in two different ways.

_{n}= a

_{1}+ (a

_{1}+d) + (a

_{1}+ 2d) + . . . + a

_{n}

_{n}= a

_{n}+ (a

_{n}- d) + (a

_{n}- 2d) + . . . + a

_{1}

_{n}= (a

_{1}+ a

_{n}) + (a

_{1}+ a

_{n}) + . . . + (a

_{1}+ a

_{n}) or 2S

_{n}= n(a

_{1}+ a

_{n})

02/28/18

Mark M.

02/16/18