If a^2 - b^2 = 8 and a*b = 2, find a^4 + b^4.

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We have a^2 - b^2 = 8 and ab = 2

Now (a - b)^2 = a^2 + b^2 + 2ab

=> (a^2 - b^2)^2 = a^4 + b^4 - 2*a^2*b^2

a^4 + b^4

=> (a^2 - b^2)^2 + 2*a^2*b^2

=> 8^2 + 2* (ab)^2

=> 64 + 2*2^2

=> 64 + 8

=> 72

Therefore a^4 + b^4 = 72

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Given that:

a^2 - b^2 = 8 ............(1)

ab = 2..................(2)

We need to determine a^4 + b^4

Let us square equation (1).

==> (a^2 - b^2)^2 = 8^2

==> a^4 - 2a^2 b^2 + b^4 = 64

==> a^2 + b^4 = 64 + 2(ab)^2

But from (2) we know that ab= 2

==> a^4 + b^4 = 64 -+2(2^2)

==> a^4 + b^2 = 64 + 8 = 72

==> a^4 + b^4 = 72

Approved by eNotes Editorial Team

Posted on

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