In today's blog, I bring together some of the results that I presented earlier to determine the derivative for sine and cosine.
Lemma 1: sin(A+B) - sin(A - B) = 2*cos(A)sin(B)
Proof:
(1) sin(A+B) = cos(B)*sin(A) + cos(A)*sin(B) [See here for proof]
(2) sin(A-B) = sin(A+(-B)) = cos(-B)*sin(A) + cos(A)*sin(-B) =
(3) since cos(-B) = cos(B) [See here] and sin(-B) = -sin(B) [See here], we get:
sin(A-B) = cos(B)*sin(A) - cos(A)*sin(B)
(4) sin(A+B) - sin(A-B) =
= cos(B)*sin(A) + cos(A)*sin(B) - cos(B)*sin(A) + cos(A)*sin(B) =
= 2*cos(A)*sin(B)
QED
Lemma 2: sin P - sin Q = 2*cos [(P+Q)/2 ] * sin[(P-Q)/2]
Proof:
(1) Let A = (P+Q)/2
(2) Let B = (P-Q)/2
(3) A+B = (P+Q)/2 + (P-Q)/2 = (P+P+Q-Q)/2 = P
(4) A - B = (P+Q)/2 - (P-Q)/2 = (P-P+Q+Q)/2 = Q
(5) sin(P) - sin(Q) = sin(A+B) - sin(A-B) = 2*cos(A)*sin(B) [See Lemma 1 above]
(6) Putting it all together gives us:
sin(P) - sin(Q) = 2*cos(A)*sin(B) = 2*cos[(P+Q)/2]*sin[(P-Q)/2]
QED
Theorem 1: d/dx(sin x) = cos x
Proof:
(1) Let y = sin(x)
(2) dy/dx = lim (Δx → 0) [sin(x + Δx) - sin(x)]/Δx
(3) From Lemma 2 above we know that:
sin(x+Δx) - sin(x) = 2*cos[(x+Δx+x)/2]*sin[(x+Δx-x)/2] =
= 2*cos(x + Δx/2)*sin(Δx/2)
(4) So, substituting (3) into (2) gives us:
dy/dx = lim(Δx → 0) [ 2*cos(x+Δx/2)*sin(Δx/2) ]
(5) Using the Product Law (see here), we know that:
lim(Δx → 0) [ 2*cos(x+Δx/2)*sin(Δx/2) ] =
lim(Δx → 0)[cos(x+Δx/2)] * lim(Δx → 0)[sin(Δx/2)/(Δx/2)]
(6) Now, if we set θ = Δx/2, we know that:
lim (θ → 0) [ sin(θ)/θ ] = 1 (See here for proof)
(7) We also know that
lim(Δx → 0)[cos(x + Δx/2)] = cos x
(8) This then gives us:
dy/dx = cos x * 1 = cos x.
QED
Lemma 3: cos(A+B) - cos(A-B) = -2*sin(A)sin(B)
Proof:
(1) cos(A+B) = cos(A)*cos(B) - sin(A)*sin(B) [See here for proof]
(2) cos(A-B) = cos(A+(-B)) = cos(A)*cos(-B)-sin(A)*sin(-B)
(3) Since cos(-B) = cos(B) and sin(-B) = -sin(B) [See here], we have:
cos(A-B) = cos(A)*cos(B)+sin(A)*sin(B)
(4) cos(A+B) - cos(A-B) =
=cos(A)*cos(B) - sin(A)*sin(B) - cos(A)*cos(B) - sin(A)*sin(B) =
= -2*sin(A)*sin(B)
QED
Lemma 4: cos P - cos Q = -2*sin[(P+Q)/2]*sin[(P-Q)/2]
Proof:
(1) Let A = (P+Q)/2
(2) Let B = (P-Q)/2
(3) A + B = (P+Q)/2 + (P-Q)/2 = (P+Q+P-Q)/2 = P
(4) A - B = (P+Q)/2 - (P-Q)/2 = (P + Q - P + Q)/2 = Q
(5) cos(P) - cos(Q) = cos(A+B) - cos(A-B) = -2*sin(A)*sin(B) [From Lemma 3 above]
(6) So,
cos(P) - cos(Q) = -2*sin(A)*sin(B) = -2*sin[(P+Q)/2]*sin[(P-Q)/2]
QED
Theorem 2: d/dx(cos x) = -sin x
Proof:
(1) Let y = cos(x)
(2) dy/dx = lim(Δx → 0) [ cos(x + Δ x) - cos(x) ]/Δx
(3) Now, from Lemma 4 above:
cos(x +Δx) - cos(x) = -2*sin[(x + Δx + x)/2]*sin[(x+Δx-x)/2] =
= -2*sin(x + Δx/2)*sin(Δx/2)
(4) Once again, applying the Product Rule for limits gives us:
dy/dx = lim(Δx → 0) [ cos(x + Δ x) - cos(x) ]/Δx =
lim(Δx → 0)[ -sin(x + Δx/2) ] * lim(Δx → 0) [ sin (Δx/2)/(Δx/2)]
(5) Again, setting θ = Δx/2 gives us:
lim(Δx → 0)[ sin(θ)/θ ] = 1 [See here for proof]
(6) We can also see that:
lim(Δx → 0)[ -sin(x + Δ x/2) ] = -sin(x)
(7) Putting this all together gives us:
dy/dx = -sin(x)*1 = -sin(x)
QED
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