STEP II 1994 question 3 solution - The Student Room
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STEP II 1994 question 3 solution

TSR Wiki > Study Help > Subjects and Revision > Mathematics > STEP > STEP II 1994 question 3 solution


f(0) = 1 \newline
f(x-y) = f(x)f(y) - f(a-x)f(a+y) \newline
\newline
\textb{Let;} x = a, y = 0\newline
f(a) = f(a)f(0) - f(0)f(a) = 0


i)


f(x-y) = f(x)f(y) - f(a-x)f(a+y) \newline
\textb{So:} f(a-x) = f(a)f(x) - f(0)f(a+x)\newline
f(a-x) = -f(a+x)

By symmetry f(a+y) = -f(a-y)

f(y-x) = f(x)f(y) - f(a-y)f(a+x)\newline
= f(x)f(y) - [-f(a+y)][-f(a-x)]\newline
f(y-x) = f(x-y)\newline
f(t) = f(-t)


ii)


f(2a) = f(a - (-a)) f(a)f(-a) - [f(0)]^2\newline
= - (1)^2\newline
= -1


iii)


f(2a - t) = f(2a)f(t) - f(-a)f(a+t)\newline
= -f(t) - f(a)f(a+t)\newline
= -f(t)


iv)


f(4a) = f(2a - (-2a)) = f(2a)f(-2a) - f(-a)f(a)\newline
= 1 - f(a)f(a)\newline
= 1

f(3a) = f(2a - (-a)) = f(2a)f(-a) - f(-a)f(0)\newline
= 0

f(4a + t) = f(4a - (-t)) = f(4a)f(-t) - f(-3a)f(a-t)\newline
= f(t) - f(3a)\newline
= f(t)


f(y-x) = f(x)f(y) - f(a-x)f(a+y)\newline
\newline
f = \cos(t)
\newline
 \cos(y-x) = \cosx \cosy + \sinx \siny

But  \sinx = \cos(\frac{\pi}{2} - x) and \siny = -\cos(\frac{\pi}{2} + x) so  \cos(y-x)=-\sin(\frac{\pi}{2}-x)\sin(\frac{\pi}{2}+x)

f = cos(Bt + C)

When t = -2, f = 0

cos(C - 2B) = 0

Take for example C - 2B = \frac{\pi}{2}

When t = 0, f = 1 cos(C) = 1 \newline
C = 0\newline
B = -\frac{\pi}{4}\newline
f = \cos(-\frac{\pi}{4}t) = \cos(\frac{\pi}{4}t)


f(x-y) = \cos(x\frac{\pi}{4} - y\frac{\pi}{4}) = \cos(x\frac{\pi}{4})\cos(y\frac{\pi}{4}) + \sin(x\frac{\pi}{4})\sin(y\frac{\pi}{4})


But
Unparseable or potentially dangerous latex formula. Error 5: Image dimensions are out of bounds: 1626x23
\sin(y\frac{\pi}{4}) = \cos(\frac{\pi}{2} - y\frac{\pi}{4}) = \cos[(\frac{\pi}{4})(2 - y)] = \cos[(\frac{\pi}{4})(-2 + y)] = f(a+y)\newline

\sin(x\frac{\pi}{4}) = -\cos(\frac{\pi}{2} + x\frac{\pi}{4}) = - \cos[(\frac{\pi}{4})(2 + x)] = - \cos[(\frac{\pi}{4})(-2 - x)] = -f(a-x)\newline

f(x-y) = f(x)f(y) - f(a-x)f(a+y)
as required.


Solution by Speleo

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