Question

find the sum of 18 terms of 57+49+41+...​

Answer 1

ANSWER:

FIRST TERM 'a' = 57

COMMON DIFFERENCE 'd' = 49-57

= -8

$sum \: of \: 1st \: \: 18th \: terms = \frac{n}{2} (2a + (n - 1)d) \\ s_{18} = \frac{18}{2} (2 \times 57 + 17 \times ( - 8)) \\ s_{18} = 9(114 - 136) \\ s_{18} = - 198$

HENCE THE SUM IS -198

HOPE IT HELPED YOU ☺