MCQ
If ${2^x} + {2^y} = {2^{x + y}}$, then ${{dy} \over {dx}} = $
  • A
    ${2^{x - y}}{{{2^y} - 1} \over {{2^x} - 1}}$
  • ${2^{x - y}}{{{2^y} - 1} \over {1 - {2^x}}}$
  • C
    ${{{2^x} + {2^y}} \over {{2^x} - {2^y}}}$
  • D
    None of these

Answer

Correct option: B.
${2^{x - y}}{{{2^y} - 1} \over {1 - {2^x}}}$
b
(b) On differentiating ${2^x}\log 2 + {2^y}\log 2.\frac{{dy}}{{dx}}$

$ = {2^x}{.2^y}\frac{{dy}}{{dx}}.\log 2 + {2^y}{.2^x}\log 2$

==> ${2^x} + {2^y}\frac{{dy}}{{dx}} = {2^{x + y}}\frac{{dy}}{{dx}} + {2^{x + y}}$

==> $\frac{{dy}}{{dx}} = \frac{{{2^{x + y}} - {2^x}}}{{{2^y} - {2^{x + y}}}} = {2^{x - y}}\frac{{{2^y} - 1}}{{1 - {2^x}}}$.

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