Question

which term of the Ap.. 2,7,12, is 76​

Answer 1

Answer:

Given A.P. is -2, -7, -12, ....-77

Here, a = -2 , a

N

= -77

d

1

=−7−(−2)=−7+2=−5

d

2

=−12−(−7)=−12+7=−5

Now a

n

=−77

⇒a+(n−1)d=−77

⇒−2+(n−1)(−5)=−77

⇒−[2+(n−1)5]=−77

⇒(2+5n−5)=77

⇒5n=77+3

⇒n=

5

80

⇒n=16

So, the 16th term will be -77

Now S

n

=

2

n

[2a+(n−1)d]

⇒S

16

=

2

16

[2(−2)+(16−1)(−5)]

=8[−4−15×5]=8[−4−75]

=8[−79]=−632

Hence, the sum of the given A.P. upto the term -77 is -632