Find the Foci (x^2)/21+(y^2)/121=1

$\frac{x^{2}}{21} + \frac{y^{2}}{121} = 1$

Step 1

Simplify each term in the equation in order to set the right side equal to $1$ . The standard form of an ellipse or hyperbola requires the right side of the equation be $1$ .

$\frac{x^{2}}{21} + \frac{y^{2}}{121} = 1$

Step 2

This is the form of an ellipse. Use this form to determine the values used to find the center along with the major and minor axis of the ellipse.

$\frac{{(x - h)}^{2}}{b^{2}} + \frac{{(y - k)}^{2}}{a^{2}} = 1$

Step 3

Match the values in this ellipse to those of the standard form. The variable $a$ represents the radius of the major axis of the ellipse, $b$ represents the radius of the minor axis of the ellipse, $h$ represents the x-offset from the origin, and $k$ represents the y-offset from the origin.

$a = 11$

$b = \sqrt{21}$

$k = 0$

$h = 0$

Step 4

Find $c$ , the distance from the center to a focus.

Find the distance from the center to a focus of the ellipse by using the following formula.

$\sqrt{a^{2} - b^{2}}$

Substitute the values of $a$ and $b$ in the formula.

$\sqrt{{(11)}^{2} - {(\sqrt{21})}^{2}}$

Simplify.

Raise $11$ to the power of $2$ .

$\sqrt{121 - {(\sqrt{21})}^{2}}$

Rewrite ${\sqrt{21}}^{2}$ as $21$ .

Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{21}$ as $21^{\frac{1}{2}}$ .

$\sqrt{121 - {(21^{\frac{1}{2}})}^{2}}$

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$\sqrt{121 - 21^{\frac{1}{2} \cdot 2}}$

Combine $\frac{1}{2}$ and $2$ .

$\sqrt{121 - 21^{\frac{2}{2}}}$

Cancel the common factor of $2$ .

Cancel the common factor.

$\sqrt{121 - 21^{\frac{2}{2}}}$

Rewrite the expression.

$\sqrt{121 - 21^{1}}$

Evaluate the exponent.

$\sqrt{121 - 1 \cdot 21}$

Simplify the expression.

Multiply $- 1$ by $21$ .

$\sqrt{121 - 21}$

Subtract $21$ from $121$ .

$\sqrt{100}$

Rewrite $100$ as $10^{2}$ .

$\sqrt{10^{2}}$

Pull terms out from under the radical, assuming positive real numbers.

$10$

Step 5

Find the foci.

The first focus of an ellipse can be found by adding $c$ to $k$ .

$(h, k + c)$

Substitute the known values of $h$ , $c$ , and $k$ into the formula.

$(0, 0 + 10)$

Simplify.

$(0, 10)$

The first focus of an ellipse can be found by subtracting $c$ from $k$ .

$(h, k - c)$

Substitute the known values of $h$ , $c$ , and $k$ into the formula.

$(0, 0 - (10))$

Simplify.

$(0, - 10)$

Ellipses have two foci.

${Focus}_{1}$ : $(0, 10)$

${Focus}_{2}$ : $(0, - 10)$

${Focus}_{1}$ : $(0, 10)$

${Focus}_{2}$ : $(0, - 10)$

Step 6

Find the Foci (x^2)/21+(y^2)/121=1

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