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Numbers that are next to brackets can be multiplied by everything inside those brackets, so:
3(f+3) —> (3 x f )+ (3 x 3 )and (2 x f ) + ( 2 x 6) 
Which makes:
3f + 9 + 2f + 12 
Then you add the ‘f’s together and numbers together:
5f + 21 


Maths

Expand & simplify3(f+3)+2(f+6)

You can multiply 2x with everything inside the brackets to get:

(2x x 1/2x) + (2x x 1/4)
= (x^2) + (1/2x) 
= x^2 + x/2

Hope this helps. 

How would you work out 2x(1/2x + 1/4) to get the answer x^2 + x/2?

Environment is constantly changing, forcing species to adapt.

Biology

The process of natural selection throughout the history of life on Earth has resulted in the success of some species and the extinction of other species. Why has natural selection not resulted in the existence of a single best-adapted species?

If they’re polar, then they cannot pass the lipophilic cell membrane. They are able to go through extracellular protein channels. 

Do polar hormone molecules pass directly through cell membranes and bind to intracellular receptors?

Need help on cells for anatomy and physiology

Lysis and Centrifugation. Detergent is used to break up the membrane, then cells are spun at extremely high speeds to obtain different sized fractions, one fraction will contain debris and cell membrane, another fraction contains proteins, etc. and one will contain nuclei.

How can you isolate nuclei from cells?

Where your blood gets filtered in your kidneys.

Chemistry

What is glomerular filtrate?

Artificial selection is when humans decide certain characteristics they want to in animals, e.g. high milk production in cows, so they breed them (highest milk-producing cows) together. Natural selection is when nature forces living organisms to adapt to their environment, and the best-fit (e.g. giraffes with tall necks that can reach tall branches of trees for food, or bacteria that has adapted to surviving antibiotics), survives and passes on their genes to their offspring so future generations have selected genes which are advantageous in the evolutionary perspective. 

How is artificial selection different from natural selection?

Valves are used to prevent back-flow of blood. The pulmonary vein delivers blood from the heart to the lungs which is a short distance; the heart can pump blood efficiently with no back-flow so valves are not needed. Also, the pressure is much lower in veins (compared to arteries) which also means no back-flow of blood.

Eman Khalil

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Eman Khalil