Thursday, 7 November 2013

trigonometry - Solve: cos(theta+25circ)+sin(theta+65circ)=1



This afternoon I've been studying the pythagorean identities & compound angles. I've got a problem with a question working with 2 sets of compound angles:




Solve, in the interval 0θ360, cos(θ+25)+sin(θ+65)=1





I've attempted expanding but reach a point with no common factors & see how to manipulate the trig ratios to move on; is there a solution without expanding?



cosθcos25sinθsin25+sinθcos65+sin65cosθ=1



cosθ(cos25+sin65)+sinθ(cos65sin25)=1



Could you tell me if I've made a mistake or how I could continue;
thanks




coffee is wearing out


Answer



"Solve, in the interval 0≤θ≤360, cos(θ+25)+sin(θ+65)=1"



cos(θ+25)+cos(25θ)=1



sin(θ+65)=cos(90(θ+65))=cos(25θ)



cos(θ)cos(25)sin(θ)sin(25)+cos(25)cos(θ)+sin(θ)sin(25)=1




2cos(θ)cos(25)=1



cos(θ)=12cos(25)



θ55.6°,326.5°


No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find limh0sin(ha)h without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...