JEE 2025 — Mathematics PYQ
JEE | Mathematics | 2025Let a1,a2,a3,… be a GP of increasing positive terms. If a1a5=28 and a2+a4=29, then a6 is equal to:
Choose the correct answer:
- A.
628
- B.
526
- C.
784
(Correct Answer) - D.
812
784
Explanation
a1, a2, ... GP
a1a5=28 and a2+a4=29
Let a be the first term and r be the common ratio.
a(ar4)=28 and ar+ar3=29
a2r4=28
ar2=27
⇒r27+27r=29
⇒27r2−29r+27=0
⇒27r2−28r−r+27=0
⇒27r(r−27)−1(r−27)=0
⇒(r−27)(27r−1)=0
⇒r=27, 271
∵ Increasing GP⇒r=27
∴ a=272
∴ a6=ar5=(27)4=(28)2=784
Explanation
a1, a2, ... GP
a1a5=28 and a2+a4=29
Let a be the first term and r be the common ratio.
a(ar4)=28 and ar+ar3=29
a2r4=28
ar2=27
⇒r27+27r=29
⇒27r2−29r+27=0
⇒27r2−28r−r+27=0
⇒27r(r−27)−1(r−27)=0
⇒(r−27)(27r−1)=0
⇒r=27, 271
∵ Increasing GP⇒r=27
∴ a=272
∴ a6=ar5=(27)4=(28)2=784

