Lesson 4 - Perimeter of a Triangle #6

[One speaker] 

[question: A triangle has sides with lengths, a, b, and c. The length of side a is 15 inches, and side b is two more inches than side a. If the perimeter of the triangle is equal to 40 inches, what is the length of side c?]

Narrator: [reads question] So let’s draw a triangle so that we can visualize this. [a triangle appears.] Here’s our triangle, we know we have sides a, b, and c. It tells us that a is equal to 15 inches. So we’re just going to start filling in what we know. So, a 15 inches. B is two more inches than side a. If a is 15 inches, b is going to be 17 inches. C is what we’re trying to find, so that’s going to be our variable. We know that the perimeter is equal to 40 inches. We also know that the perimeter of a triangle is equal to a plus b plus c. [p()= a+b+c] the perimeter of the triangle, we add up the three sides. So we can fill in what we know and solve for our variable. We know that our perimeter is 40, so we’ll write 40 over here. We know that side a is 15 inches, and we know that side b is 17 inches, and we know that if we add a, b, and c, that will equal our perimeter. [40=15+17+c] So now we can solve this and solve for c. Fifteen plus 17 is 32, and we still have our plus c over here, and that is al equal to 40. [40=32+c] Now if I’m trying to isolate c, I can subtract off 32 from both sides. Then I’m left with just c on the right side, because we have 32-32. So those will cancel out and give us zero. Then we have 40 minus 32 is 8. [8=c] So we know that our side c is equal to 8, and our units are inches. [c=8 in]          

[End of video]