How to find the length of the side of a right triangle - GRE Quantitative Reasoning

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Question

2 triangles are similar

Triangle 1 has sides 6, 8, 10

Triangle 2 has sides 5 , 3, x

find x

Answer

Draw the triangles

Triangle 1 is a 6,8,10 right triangle with 10 as the hypotenuse

Triangle 2: 3 is half of 6, 5 is half of 10; x must be half of 8

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Question

Given the diagram, indicate if Quantity A is larger, Quantity B is larger, if they are equal, or if there is not enough information given to determine the relationship.

Quantity A: $\dpi{100} \small x$

Quantity B: 7.5

Answer

Since this is a 30-60-90 triangle, we know that the length of the side opposite the 60 degree angle is $\dpi{100} \small \sqrt{3}$ times the side opposite the 30 degree angle. Thus, $\dpi{100} \small 5\sqrt{3}$ , which is about 8.66. This is larger than 7.5.

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Question

A right triangle's perimeter is $\dpi{100} \small 3+\sqrt{3}$ . The other two angles of the triangle are 30 degrees and 60 degrees.

Quantity A: The triangle's hypotenuse length

Quantity B: 2

Answer

The ratio of the sides of a 30-60-90 triangle is $\dpi{100} \small x:x{\sqrt{3}}:2x$ , with the hypotenuse being $\dpi{100} \small 2x$ . Thus, the perimeter of this triangle would be $\dpi{100} \small x+x\sqrt{3}+2x=3x\sqrt{3}$ . Since the triangle depicted in this problem has a perimeter of $\dpi{100} \small 3\sqrt{3}$ , $\dpi{100} \small x$ must equal 1, which would make the hypotenuse equal to 2.

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Question

If the shortest side of a right triangle has length and its hypotenuse has length , what is the length of the remaining side?

Answer

Use the Pythagorean theorem, $a^2 + b^2 = c^2$ , with $\dpi{100} \small a=x-4$ and $\dpi{100} \small c=x+4$ , and solve for $\dpi{100} \small b$ .

$(x-4)^2 + b^2 = (x+4)^2$

Rearrange to isolate $\dpi{100} \small b^{2}$ :

$b^2 = (x+4)^2 - (x-4)^2$

$b^2 = (x+4)(x+4) - (x-4)(x-4)$

Use FOIL to multiply out:

$b^2 = (x^2 + 8x + 16) - (x^2 - 8x + 16)$

Distribute the minus sign to rewrite without parentheses:

$b^2 = x^2 + 8x + 16 - x^2 + 8x - 16$

Combine like terms:

$b^2 = 16x$

Take the square root of both sides:

$b = 4\sqrt{x}$

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Question

Given the following triangle, what is the length of the unknown side?

Answer

At first sight, it's tempting to assume this is a right triangle and to thus use the Pythagorean Theorem to find a length of 5 for the missing side.

However, the triangle was not stated to be a right triangle in the problem statement, and no indication was given in the drawing to indicate that it was a right triangle either, such as a square demarcation in the vertex opposite the side measuring 13.

Thus there is not enough information to give the length of the missing side. When taking standardized math tests, be careful making assumptions about information that is not given.

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