Question

solve the above question​

Answer 1

Answer: 525

Step-by-step explanation:

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Answer 2

Step-by-step explanation:

21 + 23 + .......... + 49

Now , As you can see it's AP ( Arithmetic Progression ) with following :-

a = 21

d = 2

tn = 49

Now, use these values in the formula :-

tn = a + ( n - 1 ) d

49 = 21 + ( n - 1 ) 2

49 = 21 + 2n - 2

49 = 19 + 2n

49 - 19 = 2n

30 = 2n

30 / 2 = n

Therefore , n = 15

Now,

S 15 = 15 / 2 ( 2 ( 21 ) + ( 15 - 1 ) 2 )

= 15 / 2 ( 42 + 28 )

= 15 / 2 ( 70 )

= 15 × 35

= 525

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