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## Homework Statement

x=sin(a) / cos(b)

a+b < pi/2

a>0, b>0

show that

da/dx = cos^3(b)cos(a) / cos(a+b)cos(a-b)

## The Attempt at a Solution

dx/da = cos(a) / cos(b) therefore

da/dx = cos(b) / cos(a)

=cos^3(b)cos(a) / cos^2(b)cos^2(a)

However the denominator of the desired format = cos(a+b)cos(a-b) = cos^2(a)cos^2(b)-sin^2(a)sin^2(b)

Not sure how to get rid of the sin^2(a)sin^2(b) term...