Added Surplus = (2 x 4) + (1/2 x 2 x 2 ) = $10
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Suppose the price of "pre-viewed" tapes is $3, so she buys 6 tapes. Consumer surplus is greater at the lower price. At the lower price she gains greater surplus on the 4 tapes she would have bought at $5, plus she gains surplus from the 5th and 6th tapes.
The blue area is the additional consumer surplus she enjoys when movies are $3 instead of $5. Her total consumer surplus is the blue area + the green area. If we compute the area of this new larger surplus we have:
CS = 1/2 x 6 x 6 = $18
We can compute the value of the additional surplus too by recognizing that it is a rectangle and a small triangle:
Added Surplus = (2 x 4) + (1/2 x 2 x 2 ) = $10
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