Just completed the Keynes 45˚ line (still in the CIE A2 syllabus) with my A2 class and find this graph useful to explain it. A popular multi-choice question and usually in one part of an essay. Make sure that you are aware of the following;
1. C and S are NOT parallel
2. The income level at which Y=C is NOT the equilibrium level of Y which occurs where AMD crosses the 45˚ line.
1. OA is autonomous consumption.
2. Any consumption up to C=Y must be financed.
3. At OX1 all income is spent
4. At OB consumption = BQ and saving= PQ
5. Equilibrium level of Y shown in 2 ways
a) where AMD crosses 45˚ line
b) Planned S = Planned I – point D
Remember the following equilibriums:
2 sector – S=I
With Govt – S+T = I+G
With Govt and Trade – S+T+M = I+G+X