Re: Quadratic Substitution Problem
f(x) = 8x^(7/10) - x^(9/20) - 69x(1/5) = x^(1/5) [8x^(1/2)-x^(1/4)-69] = 0
Let u = x^(1/4), then u^2= x^(1/2), ergo 8u^2-u-69 = (u-3)(8u+23) = 0
u = 3 implies 3 = x^(1/4), x =81, u = -23/8 implies -23/8 = x^(1/4), a contradiction.
Hence x = 81 or x =0 [x^(1/5) = 0, x = 0} see first line.
