Question

Find the sum of first 20 termsof AP 3,7,11,15...... using formula

Answer 1

Answer:

820

Step-by-step explanation:

a = 3

d = t2 - t1

= 7 - 3

= 4

tn = a + ( n - 1 ) d

= 3 + ( 20 -1 ) 4

= 3 + ( 19 ) 4

= 3 + 76

= 79

Sn = n × [ t1 + tn ]

2

= 20 × [ 3 + 79]

2

= 10 × 82

= 820

therefor , Sum of first 20 terms is 820

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