how to solve The width of a triangle is one half its length. The perimeter of the rectangle is 54cm. What are the width and the length of the rectangle ?
Do you mean the width of a rectangle is one half it's length? If so,
2W = L (1/2 of width is same as twice the length)
2W + 2L = 54 (formula for the perimeter of a rectangle)
2(1/2L) + 2L = 54 (substitude using your formula above about width 1/2 of length)
L + 2L = 54 (simplify)
3L = 54 (simplify)
L = 9
W = 18