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If the perimeter is 42 inches, then the length of the third side is

42 inches - (10 inches + 18 inches), or 42 inches less 28 inches, or 14 inches.

There's a "Heron's Formula" which is used in finding the area of a triangle when the lengths of the 3 sides are known (but the angles are not known).  Please look up "Heron's Formula."

You could also find the measure of any one of the angles by using the Law of Cosines.  Once you have the measure of one angle, you can find the remaining angles using the Law of Sines.  Please look up these Laws.

It's up to you to decide which method to apply here.
                                                                                  a+b+c
If you choose Heron's Formula, then calculate S = ----------- , where
                                                                                       2
a=10, b=14 and c=18 (inches)

and then evaluate the following:

Area of triangle = sqrt( S(S-a)(S-b)(S-c) )

Hint:  S = 21 inches in this problem.

The area of the triangle will be 69.65 square inches.

What is the triangle?

A triangle is a three-sided polygon with three angles. The angles of the triangle add up to 180 degrees.

The two sides of the triangle are 10 inches and 18 inches long.

And the perimeter of triangle is 42 inches.

Then the third side of the triangle will be

Perimeter = Sum of all sides

           42 = 10 + 18 + Third side

Third side = 14

The area of the triangle is given as,

[tex]\rm Area = \sqrt{s(s - a)(s - b)(s - c)}[/tex]

The value of s is the half of perimeter.

s = P / 2

s = 42 / 2

s = 21

Then the area of the triangle will be

[tex]\rm Area = \sqrt{21(21 - 10)(21 - 18)(21 - 14)}\\[/tex]

Simplify the expression, then we have

Area = √4851

Area = 69.65 square inches

Thus, the area of the triangle will be 69.65 square inches.

More about the triangle link is given below.

https://brainly.com/question/25813512

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