Question

if in an ap a7 =108 a11=212 then find sum of 15 terms

Answer 1

please help me with the kids and I are

Answer 2

Heya !!



A7 = 108




A + 6D = 108 ----------(1)



And,




A11 = 212


A + 10D = 212 ---------(2)




From equation (1) we get,




A + 6D = 108



A = ( 108 - 6D ) ----------(3)


Putting the value of the A in equation (2)



A + 10D = 212



108 - 6D + 10D = 212




4D = 212 - 108



4D = 104



D = 104/4


D = 26




Putting the value of D in equation (3)



A = ( 108-6D ) = ( 108 - 6 × 26 )



A = ( 108 - 156) = -48





First term ( A ) = -48


And,


Common difference ( D ) = 26.



Sn = N/2 × [ 2A + ( N - 1 ) × D ]



S15 = 15/2 × [ 2 × -48 + ( 15 - 1 ) × 26 ]





=> 15/2 × ( -96 + 364 )





=> 15/2 × ( 268)




=> ( 15 × 134 )



=> 2010.