Question

[dream XD]

Tìm x :

a) \(1+3+5+7+...+\left(2x-1\right)=750\) 

b) \(1+5+9+13+17+...+x=501501\) 

Answer 1

[Akai Haruma]

Lời giải:

a) 

$1+3+5+...+(2x-1)=750$

$\frac{(2x-1+1)x}{2}=750$

$x^2=750$

$x=\sqrt{750}$ (vô lý?!!)

b) 

$1+5+9+13+...+x=501501$

$\frac{1}{2}(\frac{x-1}{4}+1)(x+1)=501501$

$(x+1)(x+3)=4012008$$x(x+4)=4012005=2001.2005$

$\Rightarrow x=2001$