Square Trouble

Algebra Level 4

x 4 + a x 3 + b x 2 + c x + 1 x 4 + 2 a x 3 + 2 b x 2 + 2 c x + 1 \begin{aligned} x^{4}+ \ ax^{3}+ \ bx^{2}+ \ cx+1\\ x^{4}+2ax^{3}+2bx^{2}+2cx+1\\ \end{aligned}

Both the expressions above are perfect squares for some a , b , c > 0 a,b,c>0

Find a + b + c a+b+c


The answer is 7.

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1 solution

Aareyan Manzoor
Mar 7, 2015

let x 4 + a x 3 + b x 2 + c x 2 + 1 = ( x 2 + α x + 1 ) 2 x^4+ax^3+bx^2+cx^2+1= (x^2+\alpha x+1)^2 x 4 + a x 3 + b x 2 + c x 2 + 1 = x 4 + 2 α x 3 + ( α 2 + 2 ) x 2 + 2 α x + 1 x^4+ax^3+bx^2+cx^2+1=x^4+2\alpha x^3+(\alpha^2+2) x^2+2\alpha x+1 we can say { a = c = 2 α b = α 2 + 2 = a 2 4 + 2 \begin{cases} a=c=2\alpha\\ b=\alpha^2+2=\frac{a^2}{4}+2\end{cases} let x 4 + 2 a x 3 + 2 b x 2 + 2 c x 2 + 1 = ( x 2 + β x + 1 ) 2 x^4+2ax^3+2bx^2+2cx^2+1=(x^2+\beta x+1)^2 x 4 + 2 a x 3 + 2 b x 2 + 2 c x 2 + 1 = x 4 + 2 β x 3 + ( β 2 + 2 ) x 2 + 2 β x + 1 x^4+2ax^3+2bx^2+2cx^2+1=x^4+2\beta x^3+(\beta^2+2) x^2+2\beta x+1 we can say that { a = β 2 b = β 2 + 2 = a 2 + 2 \begin{cases} a=\beta\\ 2b=\beta^2+2=a^2+2\end{cases} inserting b from the first expression, 2 ( a 2 4 + 2 ) = a 2 + 2 a = 2 2(\dfrac{a^2}{4}+2)=a^2+2\longrightarrow a=2 and a + b + c = a + a + a 2 4 + 2 = 2 + 2 + 4 4 + 2 = 7 a+b+c=a+a+\dfrac{a^2}{4}+2=2+2+\dfrac{4}{4}+2=\boxed{7}

Yup. Just as I did it.

Jake Lai - 6 years, 3 months ago

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And just the same as I will say.

Kartik Sharma - 6 years, 3 months ago

The best way to get the answer

Shubhendra Singh - 6 years, 3 months ago

Awesome.......

Ak Sharma - 6 years, 3 months ago

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