Question

in an ap 9th term is 55 21st term exceeds 13th term by 48 find d and a1​

Answer 1

Answer:

Step-by-step explanation:

Let total no. of terms be 2n, & first term, a and common diff be d.

a+(2n−1)d=l...…..(1)

l−a=10.5

if we take odd terms only it will also be a A.P. of n terms only it will also be a A.P of n terms with common diff. of 2d.

24=n/2{2a+(n−1)2d}

24=n(a+(n−1)d)….(2)

30=n/2{2(a+d)+(n−1)2d}

30=n(a+nd) ….. (3)

Put the value of equation (3) in equation (2)

24=30−nd

nd=6, putting in equation (1)

12−d=10.5

d=1.5 so n=4

a=1.5

so no of terms 8.

series 1.5,3,4.5,6,7.5,9,10.5,12