Explain help #1 ​

1.  Sum  =   ( First term + Last term) * (number of terms) / 2  =

(44 + 26) * (7/2)  =

(70) / 2 * 7 =

35 * 7  =

245

2. Similarly

(7.6 +   -1.5) * 8 /2  =

(6.1) * 4  =

24.4