Question

in the first and the fifth term of an ap are 4 and 20 respectively find the common difference​

Answer 1

Step-by-step explanation:

Let a is first term and d is common difference of an A.P.

a=4

tn=a+(n-1)d

t5=4+(5-1)d

20=4+4d

4d=20-4

4d=16

d=16/4

d=4

Therefore,the common difference is 4.

Answer 2

Step-by-step explanation:

T5=-4

t12=-18

so,

a+4d=-4.........1)

a+11d=-18.......2)

on subtracting 1 from 2 we have:

7d=-14

d=-2

on putting value of d in eq 1 we have:

a=4

now,

S20=20/2{2a+(20-1)d}

=10{8-38}

=10{-30}

=-300