Question

) Complete the following activity to find the sum of all natural numbers between
1 and 81 divisible by 6.​

Answer 1

Answer:

6,12,18---------------------------78

a = 6

d = 6

an = 78

an = a+(n-1) d

78 = 6+(n-1) 6

72 = (n-1)6

12 = n-1

n = 13

s13 = 13/2(12+(13-1)6)

13/2×(12+72)

13/2×84

13×84/2

13×42

546

Answer 2

Answer:

546

Step-by-step explanation:

6,12,18,24,30,36,42,48,...78 is an AP(multiples of 6 btwn 1 and 81)

n=13 a=6 d=6

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

S13= 13/2[ 2×6 +(13-1) 6]

=13/2 [12+72]

=13/2×84

= 13×42

=546