X2+y2=42

Solution for X2+y2=42 equation:

X2+X2=42

We move all terms to the left:

X2+X2-(42)=0

We add all the numbers together, and all the variables

2X^2-42=0

a = 2; b = 0; c = -42;
Δ = b2-4ac
Δ = 02-4·2·(-42)
Δ = 336
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{336}=\sqrt{16*21}=\sqrt{16}*\sqrt{21}=4\sqrt{21}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-4\sqrt{21}}{2*2}=\frac{0-4\sqrt{21}}{4}
=-\frac{4\sqrt{21}}{4}
=-\sqrt{21}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+4\sqrt{21}}{2*2}=\frac{0+4\sqrt{21}}{4}
=\frac{4\sqrt{21}}{4}
=\sqrt{21}
$