Algebra 02

Algebra Level 4

1 2 ( a + b α + c α 2 + d α 3 ) + 1 2 ( a + b β + c β 2 + d β 3 ) \dfrac12 (a + b\alpha + c\alpha^2 + d\alpha^3 ) + \dfrac12 (a + b\beta + c\beta^2 + d\beta^3)

If α \alpha and β \beta are roots to the equation 6 x 2 6 x + 1 = 0 6x^2-6x+1 = 0 , and the value of the expression above can be expressed as a p + b q + c r + d s \dfrac ap + \dfrac bq + \dfrac cr + \dfrac ds for integers p , q , r p,q,r and s s , find the value of r s p q \dfrac{ r\cdot s}{p \cdot q} .


The answer is 6.

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2 solutions

Raj Rajput
Jan 29, 2016

That Calligraphy Kills me everytime.

Kushagra Sahni - 5 years, 4 months ago

Nice solution man

Achal Jain - 5 years, 4 months ago

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Thanks @achal jain :) :)

RAJ RAJPUT - 5 years, 4 months ago

Woah! Nice problem + solution again!

Pi Han Goh - 5 years, 4 months ago

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Thanks @Pi Han Goh :) :)

RAJ RAJPUT - 5 years, 4 months ago

Newton's sums made it easy !!

Akshat Sharda - 5 years, 4 months ago
Wissam Akil
Jan 30, 2016

We will first start by finding the roots of the equation : 6 x 2 6 x + 1 = 0 6x^2-6x+1=0 and we do so by the "Quadratic formula" : x = b ± b 2 4 a c 2 a x=\frac{-b\pm \sqrt{b^2-4ac}}{2a} where a = 6 , b = 6 , c = 1 a=6 , b=-6 , c=1

we therefore get that the two roots are 3 + 3 6 , 3 3 6 \frac{3+\sqrt{3}}{6} , \frac{3-\sqrt{3}}{6} and we can say that α = 3 + 3 6 , β = 3 3 6 \alpha =\frac{3+\sqrt{3}}{6}, \beta =\frac{3-\sqrt{3}}{6} now the problem is simple , all we have to do is rearrange the equation , and equate coefficients : 1 2 ( a + b α + c α 2 + d α 3 ) + 1 2 ( a + b β + c β 2 + d β 3 ) = a + 1 2 b ( α + β ) + 1 2 c ( α 2 + β 2 ) + 1 2 d ( α 3 + β 3 ) \frac{1}{2}(a+b\alpha +c\alpha ^2+d\alpha ^3)+\frac{1}{2}(a+b\beta +c\beta ^2+d\beta ^3) =a+\frac{1}{2}b(\alpha +\beta )+\frac{1}{2}c(\alpha ^2+\beta^2 )+\frac{1}{2}d(\alpha ^3+\beta ^3) and this equation can be written as : a p + b q + c r + d s \frac{a}{p}+\frac{b}{q}+\frac{c}{r}+\frac{d}{s} therefore by equating coefficients one finds : 1 p = 1 , 1 q = α + β 2 , 1 r = α 2 + β 2 2 , 1 s = α 3 + β 3 2 \frac{1}{p}=1 , \frac{1}{q}=\frac{\alpha +\beta }{2} , \frac{1}{r}=\frac{\alpha ^2+\beta ^2}{2},\frac{1}{s}=\frac{\alpha ^3+\beta ^3}{2} and by computing these values one finds : p = 1 , q = 2 , r = 3 , s = 4 p=1 , q=2 , r=3 , s =4 Therefore r . s p . q = 3 4 1 2 = 12 2 = 6 \frac{r.s}{p.q}=\frac{3*4}{1*2}=\frac{12}{2}=6

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