Question

please answer
I will mark as brainlist
I will follow too​

Answer 1

Answer:

a)210

b)1050

Step-by-step explanation:

a)1+2+3+4+5+.......+20

sol:Sum of 'n' Natural numbers=n(n+1)/2

Therefore: here we have natural numbers upto1 to 20 ,So,Apply the formula n(n+1)/2.

1+2+3+4+5......+20=

n=number of natural numbers.

Therefore:

20(20+1)/2=20(21)/2

=5(21)/1

=210

Apply the same formula.

b)5+10+15+20+25+.....+100

n(n+1)/2=5+10+15+20+25+.......+100

here we have the multiples of 5 ,So let we bring 5 out .So, 5(1+2+3+4+5+.....+20)

Apply the formula:taking 5 out

=n(n+1)/2=20(20+1)/2

= 10(21)/1

= 210.

So.let's multiply with the 5 which we have taken out

=210×5=1050.

Sorry,I don't know 3rd one.Thankyou.