SOLUTION: If a+1/3a=2 then find the value of 9a^2+1/a^2

Algebra ->  Finance -> SOLUTION: If a+1/3a=2 then find the value of 9a^2+1/a^2      Log On


   



Question 1206122: If a+1/3a=2 then find the value of 9a^2+1/a^2
Found 2 solutions by greenestamps, math_tutor2020:
Answer by greenestamps(13203) About Me  (Show Source):
You can put this solution on YOUR website!


The given equation is a%2B1%2F3a=2.

Square both sides of the equation:

%28a%2B1%2F3a%29%5E2=2%5E2
a%5E2%2B2%2F3%2B1%2F9a%5E2=4

Note that the middle term when squaring the left side of the equation is a constant; it contains no variables. That is the key to solving a problem like this.

a%5E2%2B1%2F9a%5E2=4-2%2F3=10%2F3

Multiply both sides of the equation by 9 to get the expression we want:

9a%5E2%2B1%2Fa%5E2=9%2810%2F3%29=30

ANSWER: 30


Answer by math_tutor2020(3817) About Me  (Show Source):
You can put this solution on YOUR website!

I'll assume the given equation is a+%2B+1%2F%283a%29+=+2
And I'll assume you want to find the value of 9a%5E2+%2B+1%2F%28a%5E2%29
Please let me know if otherwise.

If my assumptions are correct, then you should be careful to use parenthesis.
Something like a + 1/3a could be misinterpreted as a%2B%281%2F3%29a
So instead of a + 1/3a, you should type in a + 1/(3a)
Instead of 9a^2+1/a^2, you should type in 9a^2+1/(a^2)

The original equation just involves 'a' without any exponent.
The thing we want to find has 'a' being squared.
A naive approach could be to simply square both sides to see what happens.

a+%2B+1%2F%283a%29+=+2

%28a+%2B+1%2F%283a%29%29%5E2+=+2%5E2

a%5E2+%2B+2%2Aa%2A%281%2F%283a%29%29+%2B+%281%2F%283a%29%29%5E2+=+4 Use formula (p+q)^2 = p^2+2pq+q^2

a%5E2+%2B+2%2F3+%2B+1%2F%289a%5E2%29+=+4 The 'a's cancel out in the middle portion.

a%5E2+%2B+1%2F%289a%5E2%29+=+4+-+2%2F3

a%5E2+%2B+1%2F%289a%5E2%29+=+10%2F3

We want to find 9a%5E2+%2B+1%2F%28a%5E2%29, but we have a%5E2+%2B+1%2F%289a%5E2%29 on the left hand side.
The coefficients for the a^2 terms (1 and 9) have been swapped.
A quick easy fix is to multiply both sides by 9.

a%5E2+%2B+1%2F%289a%5E2%29+=+10%2F3

9%2A%28a%5E2+%2B+1%2F%289a%5E2%29%29+=+9%2A10%2F3

9a%5E2+%2B+9%2A%281%2F%289a%5E2%29%29+=+90%2F3

9a%5E2+%2B+1%2F%28a%5E2%29+=+30


Answer: 30