Question

if 8th term of an A.P is 15,then sum of its 15 terms is

Answer 1

t8= a+7d=15
S15=15/2[2a+14d]=15(a+7d)=15×15=225

Answer 2

hello users .......

we have given that 
8th term of an AP is 15 

we have to find 
sum of first 15  terms of AP =? 

solution :-

we know that 
$S_{n}$ = $\frac{n}{2}$ { 2a + (n-1)d}
and 
$A_{n}$ = a +(n-1)d 


here 
$A_{8}$ = 15 
=> a + 7d = 15 ..........(1)

now 
$S_{15}$ = $\frac{15}{2}$ { 2a + 14d }
=> $S_{15}$ = $\frac{15}{2}$ { 2( a + 7d ) }
= 7.5 {2 × 15 } 
= 7.5 × 30 = 225 

hence 

$S_{15}$ = 225 answer 


☆☆ hope it helps ☆☆