Math Problem Statement

Find the area of the ABC whose perimeter is 96cm and 90o  = B and 30o  = ACB . (Assume 3 1.732 = )

Solution

We are tasked with finding the area of the triangle ΔABC\Delta ABC where:

  • The perimeter is 96 cm.
  • B=90\angle B = 90^\circ.
  • ACB=30\angle ACB = 30^\circ.
  • The approximation 31.732\sqrt{3} \approx 1.732.

Step-by-Step Solution:

  1. Label the sides:
    Since B=90\angle B = 90^\circ, triangle ΔABC\Delta ABC is a right-angled triangle. Using conventional labeling, let's denote:

    • Side AC=cAC = c (the hypotenuse).
    • Side AB=aAB = a and BC=bBC = b as the two other sides of the triangle.
  2. Apply angle relationships:
    Since ACB=30\angle ACB = 30^\circ, we can use trigonometric ratios in the right triangle to express the sides.

    • Using sin(30)=12\sin(30^\circ) = \frac{1}{2}, we know: sin(30)=ABAC=aca=c2.\sin(30^\circ) = \frac{AB}{AC} = \frac{a}{c} \quad \Rightarrow \quad a = \frac{c}{2}.

    • Using cos(30)=32\cos(30^\circ) = \frac{\sqrt{3}}{2}, we also know: cos(30)=BCAC=bcb=c32.\cos(30^\circ) = \frac{BC}{AC} = \frac{b}{c} \quad \Rightarrow \quad b = \frac{c \sqrt{3}}{2}.

  3. Perimeter condition:
    The perimeter is given as 96 cm. Thus, we can write: a+b+c=96.a + b + c = 96. Substituting a=c2a = \frac{c}{2} and b=c32b = \frac{c \sqrt{3}}{2} into the equation, we get: c2+c32+c=96.\frac{c}{2} + \frac{c \sqrt{3}}{2} + c = 96. Simplifying the left-hand side: c2+1.732c2+c=96,\frac{c}{2} + \frac{1.732c}{2} + c = 96, c(1+1.732)2+c=96,\frac{c (1 + 1.732)}{2} + c = 96, c×2.7322+c=96.\frac{c \times 2.732}{2} + c = 96. Multiply both sides by 2: c(2.732+2)=192,c(2.732 + 2) = 192, c×4.732=192.c \times 4.732 = 192. Solving for cc: c=1924.73240.58cm.c = \frac{192}{4.732} \approx 40.58 \, \text{cm}.

  4. Calculate side lengths:

    • a=c2=40.582=20.29cma = \frac{c}{2} = \frac{40.58}{2} = 20.29 \, \text{cm}.
    • b=c32=40.58×1.732235.12cmb = \frac{c \sqrt{3}}{2} = \frac{40.58 \times 1.732}{2} \approx 35.12 \, \text{cm}.
  5. Area of the triangle:
    The area AA of a right triangle is given by: A=12×a×b.A = \frac{1}{2} \times a \times b. Substituting the values of aa and bb: A=12×20.29×35.1212×713.54=356.77cm2.A = \frac{1}{2} \times 20.29 \times 35.12 \approx \frac{1}{2} \times 713.54 = 356.77 \, \text{cm}^2.

Thus, the area of ΔABC\Delta ABC is approximately 356.77cm2\boxed{356.77 \, \text{cm}^2}.


Do you want more details or have any questions?
Here are 5 related questions to expand on this topic:

  1. How do we use the sine and cosine functions in right-angled triangles?
  2. What is the relationship between angles and side lengths in a 30-60-90 triangle?
  3. How would the solution change if the perimeter were different?
  4. Can the area of the triangle be found using Heron’s formula instead?
  5. What are other methods to find the area of a triangle when given angles and sides?

Tip: In any right triangle, knowing two angles allows you to determine all side lengths using trigonometric ratios like sine and cosine.

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Math Problem Analysis

Mathematical Concepts

Geometry
Trigonometry
Right Triangles

Formulas

sin(30°) = 1/2
cos(30°) = √3/2
Perimeter formula: a + b + c
Area of a right triangle: A = 1/2 × a × b

Theorems

Trigonometric Ratios
Pythagorean Theorem

Suitable Grade Level

Grades 9-12