Question

If a = 6, d = 4 then find S20-​

Answer 1

$Sn = 880.$

Step-by-step explanation:

$Given$

$a = 6. \\ d = 4. \\ n = 20.\\ S20 = {?}$

$Sn \: = \frac{n}{2} [2a + (n-1) d]$

$= \frac{20}{2} [2×6 + (20-1) 4]$

$= 10[12 + (19) 4]$

$= 10[12 + 76]$

$= 10[88]$

$Sn = 880.$