Question

find the sum of the first 20th term of the A.p 2+5+__________

Answer 1

here a=2 . c.d.= 3 therefore a20 = a + (n -1) d = 2 + (20 - 1) 3 =2 + 57 = 59

Answer 2

a20=a+19d
a=2 d=a2-a1 d=5-2=3
a20=2+19×3
=2+57 =59
now we got first and last term
sn=n÷2(a+l)
n=59 a=2 l=59
=59/2(2+59)
18(61)
=1098