Answer:
see explanation
Explanation:
Note that the formula given is incorrect.
The formula for the sum to n terms of an arithmetic sequence is
=
[ 2a + (n - 1)d ]
Substituting the given values into the formula
=
[ (2 × 50) + ( - 2 × 41) ]
= 21 ( 100 - 82)
= 21 × 18 = 378