Question

find the sums
:) I +2+3..... +20
b) 3+ 6+9+
+6+
760
c) 1/3+2/7+3 7+
+20/7​

Answer 1

Answer:

a) 1+2+3..... +20

here given:

a=1

d=2-1=1

n=20

Now, by the formula:

S(n)= n/2[2a+(n-1)d]

= 20/2[2(1)+(20-1)1]

=10[2+(19)]

=10[21]

= 210

b) 3+ 6+9+6+...+760

here given:

a=3

d=6-3=3

n=760

Now, by the formula:

S(n)= n/2[2a+(n-1)d]

= 760/2[2(3)+(760-1)3]

= 380[6+(759)3]

= 380[6+2277]

= 380[2283]

= 867540

c) 1/3+2/7+3/7+...+20/7

here given:

a=1/3

d=2/7-1/3=-1/21

n=20/7

Now, by the formula:

S(n)= n/2[2a+(n-1)d]

= 1/3/2[2(1/3)+(20/7-1)-1/21]

=2/3[2/3+(13/7)-1/21]

=2/3[2/3+(-13/147)]

= 2/3[85/147]

= 170/441

= 0.386 (approx.)

hope it helps :)