Concepts in Unit Digits for CAT QA and DI
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Unit digit concepts play a lot of time saving tricks in the CAT format.We can use the concept of unit digits in eliminating answer options in all the objective type competitive examinations.

The unit digit has got huge significance when it comes to eliminate the answer options.Lot of questions involving rigorous multiplications can be solved within no time by eliminating the answer options

CASE 1...
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Lets take the case of the product of two successive numbers,n(n+1)
We will see the possible unit digits of the products.

0*1=0
1*2=2
2*3=6
3*4=2
4*5=0
5*6=0
6*7=2
7*8=6
8*9=2

we can see that, the unit digits of products of successive numbers will end in 0,2,6 only.
Now we have the formula for sum of first n natural numbers, n(n+1)/2
So we will see the possible unit digits for these products.
We know that,n(n+1) ends in 0,2,6.
So we consider the possible digits, we will get, when we divides n(n+1) by 2.
0 gives 0,5
2 gives 1,6
6 gives 3,8
So these are the possible unit digits for the sum of first n natural numbers.

*Product of successive numbers,n(n+1) ends in 0,2,6

*The sum of first n, natural numbers will never end in 2,4,7,9

*Perfect squares never end in 2,3,7,8

*If there are n terms in an AP, the difference between sum of odd terms and sum of even terms is (n/2)*d where d is the common difference.

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