Question

Find the sum of first 20 terms of an A.P. whose
common difference is 3 times the first term &
its 5th term is 25​

Answer 1

Answer:

sn = 1146.15

Explanation:

tn=a+(n-1)d

25=a+(5-1)3a. {3a=d}

25=a+12a

25=13a

a=25/13

d=3a

d=3*25/13

d=75/13

sn =n/2 (2a+(n-1)d)

sn=20/2{2*25/13+(20-1)75/13}

sn=10{50/13+1425/13}

sn=10(50+1425/13)

sn=10(159.61)

sn=1146.15