Question

find the sum of first 12terns of AP whose nth terms is given by 3n +4​

Answer 1

Answer:Given,

n

th

term T

n

​

=1−4n

put, n=1, T

1

​

=a=1−4(1)=−3

put, n=2, T

2

​

=1−4(2)=−7

Common difference, d=T

2

​

−T

1

​

=−7−(−3)=−4

So, we have a=−3, d=−4, n=25, to find the sum.

Sum, S

n

​

=

2

n

​

(2a+(n−1)d)

S

25

​

=

2

25

​

(2(−3)+(25−1)(−4))

S

25

​

=

2

25

​

(−112)=25×(−51)

S

25

​

=−1275

This is the sum.

Step-by-step explanation: