@Carl Love Hi Carl, sorry for the mistakes, and thanks for your participation!
Let me rearrange the question of this differentiation, so I have the equation as follow
eq3 := P = 1/2*gamma*H^2*[cos(theta - beta)*cos(theta - alpha)*sin(beta - phi)/(cos(theta)^2*sin(beta - alpha)*sin(((beta + 90 + delta) - beta) + phi))]
And according to the paper I read, to get the critical value of beta for maximum value of P, I need to make differentiation to first derivative where dP/d(beta) = 0.
Note that in this case only beta is the only variables, and other coefficients are constant.
Then I should substitute back value of beta to equation above and the paper shows that i should get equation below.
P = 1/2*K*gamma*H^2
where K is equal to
K = cos(phi - theta)^2/(cos(theta)^2*cos(theta + delta)*(1 + sqrt(sin(delta + phi)*sin(phi - alpha)/(cos(theta + delta)*cos(theta - alpha))))^2)
Hope you can help me with this. It's good enough if I can learn how to perform differentiate with respect to variable.
Thank you again :)