Question

Question 15 and 16 plz I want it urgently

Answer 1

1question
a5=13
a15=-17
13=a+4d. -------(1)
-17=a+14d -------(2)
then we subtract equation (1)from (2)
-17=a+14d-(13=a+4d)
-17=a+14d-13=-a-4d
then -17-13=10d
-30=10d
-3=d
putting value of d=-3 in equation (1)we get
13=a+4*-3
13=a-12
13+12=a
25=a
then S21=n/2*2a(n-1)d
S21= 21/2*2*25*20*-3
S21= -31500








2question
a8=37=a+7d. ------------(1)
a15=15+a12
a+14d=15+a+11d
14d-11d=15
3d=15
d=5
putting value of d=5in. equation (1) SE get
37=a+7*5
37=a+35
2=a
Then AP will be a,a+d,a+2d .........
2,2+5,2+2*5......
AP= 2,7,12.....
S15=n/2*2*a*(n-1)*d
S15= 15/2*2*2*14*5
S15= 2100