(d) : Since,
2,p and
q are in G.P.\n
\n∴p2=2q#(i)\n \n\nLet first term of the A.P. be
a and common difference be
d.\n
\n\n∴T7=a+6d=2#(ii)\nT8=a+7d=p#(iii)\n\n \nand
T13=a+12d=q\n∴ From (ii) and (iii), we get
d=p−2\nFrom (ii) and (iv), we get
6d=q−2⇒6(p−2)=q−2\n
\n⇒6p=q+10#(v)\n \n\nFrom (i) and (v), we get
p=2 or
p=10∴q=2 or 50 But
q=2 Hence,
p=10,q=50∴d=8 and
a=−46\n\nSince,
5th term of G.P.
=nth term of A.P.\n
\n\n\n∴2(210)4=−46+(n−1)8⇒1250=−46+8n−8⇒8n=1304⇒n=163\n\n