 Welcome to session. I am Bhumika and will discuss with you first part of question 15th from exercise 5.1 Which is at page 90 of your book now the question says find the sum of the first hundred natural numbers here? We will be using the key idea To find the sum of the first n natural numbers If SN is the sum of the first n natural numbers then SN is equal to n by 2 2a plus n minus 1 into D or It is equal to n by 2 into a plus the last term So we'll be using this formula as of a key idea to find to solve question So we have been asked to find the sum of first hundred natural numbers Some we have to find s hundred here So we'll be using this formula SN is equal to n by 2 into 2 a plus n minus 1 into D or SN is equal to n by 2 into A plus L where a is the first term and L is the last term and here is the first term and D is the common difference So let's begin our solution What is asked we have to find s hundred This is to find and given is this natural numbers up till hundred we know The natural numbers up till hundred any number which starts from one is a natural number So all our natural numbers from after hundred after one and including one every number is a natural number So here a is one and the last term also we know so we can simply Apply this formula. So here we will be using this formula to solve this question So here s hundred is Equals to hundred by two into one plus hundred Which is equal to fifty into one zero one, which is equal to five zero five zero. So this is s hundred So hence the sum of the first hundred natural numbers is Five zero five zero, which is required. So in order to do this question What we need is very simple concept first What is a natural number any number which starts from one is a natural number and we must remember this formula Any of these formulas this question can also be derived from this formula the one first formula We know first term a we can find the common difference because you know the second term also to here common difference Will be one a is the one just find out the Just substitute the values of a and d and n to get the required so this question can be done from these two formulas both and but By doing it from second formula it will become little little less time consuming So the rest it is easy just practice formulas and just keep remember the basic things like natural numbers whole numbers and what are integers rational numbers, so that's all and That's all from the session you take care keep practicing have a nice time