SOLUTION: I can't solve this. 15x^4 - 28x^2 + 5=0 The solution is x=__

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Question 480707: I can't solve this.
15x^4 - 28x^2 + 5=0
The solution is x=__
Found 2 solutions by emargo19, MathLover1:
Answer by emargo19(101) About Me  (Show Source):
You can put this solution on YOUR website!
Let u=x%5E2 So %28u%5E2%29%5E2=x%5E4
15x%5E4+-+28x%5E2+%2B+5=0
15u%5E2-28u%2B5=0

Using the Quadratic Formula:
a=15 b=-28 c=5
x+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
So
x=+%28-%28-28%29%2B-sqrt%28%28-28%29%5E2-4%2A15%2A5%29%29%2F%282%2A15%29
x=%2828%2Bsqrt%28784-300%29%29%2F30 and x=%2828-sqrt%28784-300%29%29%2F30
x=%2828%2Bsqrt%28484%29%29%2F30 and x=%2828-sqrt%28484%29%29%2F30
x=%2828%2B22%29%2F30 and x=%2828-22%29%2F30
x=50%2F30 and x=6%2F30
x=5%2F3 and x=1%2F5
remember u+=+x%5E2 so:
x+=+sqrt%285%2F3%29+=+sqrt%2815%29%2F3 or x+=+-sqrt%285%2F3%29+=+-sqrt%2815%29%2F3
or
x+=+sqrt%281%2F5%29+=+sqrt%285%29%2F5 or x+=+-sqrt%281%2F5%29+=+-sqrt%285%29%2F5



Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!
15x%5E4+-28x%5E2+%2B+5=0..........replace -28x%5E2 with -25x%5E2-3x%5E2

15x%5E4+-25x%5E2-3x%5E2+%2B+5=0......group
5x%5E2%283x%5E2-5%29+-%283x%5E2+-+5%29=0
%285x%5E2+-1%29%283x%5E2+-5%29=0
if
5x%5E2+-1=0, then 5x%5E2+=1...->..x%5E2+=1%2F5...->..x+=sqrt%281%2F5%29
..->..x+=1%2Fsqrt%285%29..->..x+=1sqrt%285%29%2F%28sqrt%285%29sqrt%285%29%29..->..
x+=sqrt%285%29%2F5 or x+=-sqrt%285%29%2F5

if
3x%5E2+-5=0..->..3x%5E2=5..->..x%5E2=5%2F3..->..
x=sqrt%285%2F3%29 or x=-sqrt%285%2F3%29