Arectangle's length is 8cm more than three times its width it the perimeter is 128 cm find the length

The length is 50. You solve it by making an equation out of the information given. the rectangle's length(l) is 3 times its width(w) and its perimeter(p) is 128. So in order to solve this you need an equation to find the perimeter, isolating w to find l. It'll look something like this:
(3w+8)+(3w+8)+w+w=128
?You combine all like terms turning it into:
2(4w+8)=128
8w+16=128
8w=112
w=14
you use the value of w to solve for l as follows:
l=w*3+8
l=14*3+8
l=42+8
l=50

The bright field microscope can be easily converted for use with dark field illumination using special dark field condensers (dry or oil immersion).

Explanation:

In a bright field microscope, the material appears unstained. The light passes directly and the details that are colored in a natural way are appreciated. In dark field microscopy, the contrast is created by a bright specimen on a dark background. Instead of illuminating the sample with a full cone of light (as in brightfield microscopy), a special condenser is intended to produce a disguise of the light rays by sending them laterally on the object in the form of a luminous cone, and this shape also does not penetrate the target directly. This form of illumination allows only oblique rays of light to hit the sample on the microscope stage and the image is formed with the light rays scattered by the sample and captured by the objective.