Answer:
[tex]equation \ of \ exponential \ function , y = a (b)^x\\(-1, 20) \\=> 20 = a(b)^{-1}\\=>20 = \frac{a}{b} ---(1)\\\\(1, 5)\\=>5 = a(b)^{1}\\=>5=ab ---(2)\\\\from (1) a = 20b\\substitute \ a \ in \ (2)\\\\\\5 = (20b)b\\5= 20b^2\\\frac{1}{4} = b^2\\\\b = \frac{1}{2}\\substitute \ b \ in \ (2)\\5 = ab\\5 = a (\frac{1}{2})\\\\a = 10\\Therefore , the \ equation : y = 10(\frac{1}{2})^{x}[/tex]
