Answer:
Step-by-step explanation:
you know ; for all reals a ; cos(π/2 - a) = sin(a)
so : sin(5x+15) = cos(π/2-(5x+15) ) = cos(π/2-5x-15)
sin(5x+15)=cos(4x−6) eqivaut : cos(π/2-5x-15) = cos(4x−6)
note : cos(a)=cos(b) : a= b +2kπ or a= - b +2kπ ....k in Z
π/2-5x-15 = 4x−6 + 2kπ or π/2-5x-15 = - (4x−6) + 2kπ....k in Z
continue calculate : x .......
