Robinson's preferences between apples (a) and bananas (b) are expressed by the following:
U = (a+2)0.5(b+1)0.5
(a) Show that Robinson's indifference curves are negatively sloped.
(b) Are they convex to the origin? Explain.
Robinson's interference curves
a.) From U = U = (a + 2)0.5 ∗(b+ 1)0.5
Totally differentiating the utility function
"du=\\frac{\\delta}{\\delta a}[(a+2)^{0.5}(b+1)^{0.5}da]+\\frac{\\delta}{\\delta b}[(a+2)^{0.5}(b+1)^{0.5}db]"
"du=0.5(a+2)^{-0.5}(b+1)^{0.5}da+0.5(a+2)^{0.5}(b+1)^{-0.5}db"
along an indifference curve change in utility du=0
Also we may apply (a) along horizontal axis hence,
"0.5(a+2)^{-0.5}(b+1)^{0.5}da+0.5(a+2)^{0.5}(b+1)^{-0.5}db=0"
"\\frac{da}{db}=\\frac{0.5(a+2)^{0.5}(b+1)^{-0.5}}{0.5(a+2)^{-0.5}(b+1)^{0.5}}=\\frac{a+2}{b+1}"
"\\frac{da}{db}= slope \\ of\\ indifference"
"=MRS_{a,b }" [Nominal rate of substitution between a and b]
As, "MRS_{a,b } \\lt0" implies robinsons indifference curve is negatively sloped
b.) Now "|MRS_{a,b }|=\\frac{a+2}{b+1}=\\frac{a}{b+1}+\\frac{2}{b+1}"
Now "\\frac{\\delta |MRS_{a,b }|}{\\delta a}=\\frac{1}{b+1}\\gt0 \\ as\\ b\\gt0"
hence they are convex to the origin
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