JEE Main 2015 · 11 Apr · Shift 1 · Mathematics Q68

← JEE Main 2015 · 11 Apr · Shift 1

MathematicsSingle correct+4 / -1Question 68 of 90

The sum of the 3 $^{\text{rd}}$ and the 4 $^{\text{th}}$ terms of a G.P. is 60 and the product of its first three terms is 1000. If the first term of this G.P. is positive, then its 7 $^{\text{th}}$ term is :

7290

320

640

2430

Answer and worked solution

Official answer: B

$(ar)^3=1000$ , so $ar=10$ . Then $ar^2+ar^3=10r+10r^2=60$ , giving $r=2$ ( $a=5>0$ ; $r=-3$ would make $a$ negative). $T_7=5\cdot2^6=320$ .

Topics:Geometric progression

← Previous question Next question →

JEE Main 2015 · 11 Apr · Shift 1, Mathematics question 68