Posted by **Kristen** on Monday, February 4, 2013 at 7:44pm.

I have to solve each equation using a u substitution and need help understanding how to do this.

The first equation is:

1. x^2/3-2x^1/3-15=0

2. x+4=7 Squareroot x+4

- College Algebra help!!! -
**Reiny**, Monday, February 4, 2013 at 7:52pm
x^2/3-2x^1/3-15=0

(x^(1/3))^2 - 2x^(1/3) - 15 = 0

suppose we let x^(1/3) = y, then we get

y^2 - 2y - 15 = 0

(y-5)(x+3) = 0

y = 5 or y = -3

then x^(1/3)= 5 , cube both sides

x = 125

or

x^(1/3) = -3

x = -27

2.

x+4 = 7√(x+4)

square both sides

x^2 + 8x + 16 = 49(x+4)

x^2 + 8x + 16 = 49x +196

x^2 - 41x - 180=0

(x-45)(x+4) = 0

x = 45 or x = -4

BUT, since we squared, we must verify all answers

if x = 45

LS = 45+4 = 49

RS = 7√49 = 49 , works

if x = -4

LS = 0

RS = 7√0 = 0 , works

so x = 45 or x = -4

- College Algebra help!!! -
**Kristen**, Monday, February 4, 2013 at 8:03pm
Thank you so much this makes the examples provided make much more scense the way you broke them down! Thanks!

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