The fourth proportional to 4, a and 16a is 81. Find the value of a2 .
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 20.25
Let the fourth proportional = x
4 : a = 16a : x
x = (a × 16a) ÷ 4
= 16a2 ÷ 4 = 4a2
Given x = 81
4a2 = 81
a2 = 81 ÷ 4 = 20.25