SOLUTION: Please show me step by step how to factor this completely. Thanks! x^4 -10x^2 -39

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Question 928771: Please show me step by step how to factor this completely. Thanks!
x^4 -10x^2 -39
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Make a substitution, u=x^2;
u%5E2-10u-39, easily and quickly factorable into %28u%2B3%29%28u-13%29, and not further factorable, even after reversing the substitution:

%28x%5E2%2B3%29%28x%5E2-13%29