Question 732967
Rectangles have 4 right angles, a length and width. {{{drawing(300,200,-1,29,-2,18,
rectangle(0,0,28,16),locate(11,0,length),
locate(18,9,width),arrow(20,7.5,20,0),arrow(20,9,20,16),
arrow(10.5,-1,0,-1),arrow(16.5,-1,28,-1)
)}}} The length is the measure of the longest side. In the units I chose, the length of that rectangle is 7 and the width is 4.
The perimeter is {{{7+4+7+4=22}}} of my units and
the area is {{{7*4=28}}} square units
In a rectangle, if you increase the length by 1 unit and decrease the width by 1 unit, the perimeter stays the same, and the area gets smaller.The change in area depends on the original dimensions.
If I increase length of my rectangle to 8 and decrease its width to 3,
perimeter={{{8+3+8+3=22}}} and area={{{8*3=24}}}
 
Triangles do not have a "length" and "width".
You can choose one of the 3 sides as the base; set the triangle on that side; measure the length of the side called base, and measure the height perpendicular to that base. Someone may rotate the triangle and have a different base and height for the same triangle, like this:
{{{drawing(300,300,-0.7,6.3,-0.7,6.3,
triangle(0,0,5,0,3.2,2.4),triangle(0,2.5,4,2.5,0,5.5),
red(line(0,2.5,0,5.5)),red(rectangle(0,2.5,0.2,2.7)),
red(line(3.2,0,3.2,2.4)),red(rectangle(3.2,0,3.4,0.2)),
locate(1.5,2.9,base),locate(0.1,4.3,red(height)),
locate(2.15,0,base),locate(3.3,0.9,red(height)),
arrow(2,-0.2,0,-0.2),arrow(3,-0.2,5,-0.2)
)}}} There are 2 copies of the same triangle in that figure. A different side is chosen as base in each one, and the height is measured perpendicular to the chosen base.
If you increase the base by 1 unit and decrease the height by 1 unit, the perimeter and the area change in ways that depend on the original dimensions.
The triangles in my drawing have side lengths of 3, 4, and 5 of my units for that drawing.
For the top triangle I chose {{{base=4}}} , {{{height=3}}}.
For the bottom triangle {{{base=5}}} and {{{height=2.1}}}.
The perimeter of the triangle is {{{3+4+5=12}}}.
The area is {{{base*height/2}}} and can be calculated as {{{4*3/2=6}}} for the top triangle,
or {{{5*2.4/2=12}}} for the rotated version at the bottom.
If I increase each base by 1 unit and decrease the corresponding height by 1 unit, I get this:
{{{drawing(300,300,-0.7,6.3,-0.7,6.3,
triangle(0,0,6,0,3.84,1.4),triangle(0,2.5,5,2.5,0,4.5),
red(line(0,2.5,0,4.5)),red(rectangle(0,2.5,0.2,2.7)),
red(line(3.84,0,3.84,1.4)),red(rectangle(3.84,0,4.04,0.2))
)}}} The triangles are no longer the same triangle.
For the top triangle the side length are now 5, 2, and {{{sqrt(29)=about 5.385}}}.
The perimeter is about {{{5+2+5.385=12.385}}}.
The area is {{{5*2/2=5}}}.
The bottom triangle now has {{{base=6}}} and {{{height=1.4}}}
The sides measure 6, about 4.087, and about 4.406.
The new perimeter is {{{6+4.087+4.406=14.493}}}
The new area is {{{6*1.4/2=4.2}}}