Question

Q. 9 The sum of the first
20 terms of an AP in which
a = 1 and 20th term = 58​

Answer 1

Answer:

59/2

Step-by-step explanation:

a20=58,a=1,

sn=1/2(a+l)

s20=1/2(1+58)

=59/2

Answer 2

Answer:

590

Step-by-step explanation:

formula to found like this question,

n/2{2a+(n-1)d}

but, 'd' is not pre sent so there is given 20th term is 58

it means a+(n-1)d = 58

=> 1+(20-1)d= 58

: (because, a=1 , n = 20 in 20th term)

=> 19d= 58-1

=> d= 57/19 = 3

now, a = 1, & d = 3

next, the total of 1st 20th term will be

20/2{(2×1)+(20-1)3}

= 10{2+(19×3)}

= 10{ 2+57}

= 59×10 = 590