Answer:
Step-by-step explanation:
We know:
We have
Use 
![\left(\dfrac{1}{2}\right)^2+\cos^2\theta=1\\\\\dfrac{1}{4}+\cos^2\theta=1\qquad\text{subtract}\ \dfrac{1}{4}\ \text{from both sides}\\\\\cos^2\theta=\dfrac{4}{4}-\dfrac{1}{4}\\\\\cos^2\theta=\dfrac{3}{4}\to\cos\theta=\pm\sqrt{\dfrac{3}{4}}\to\cos\theta=\pm\dfrac{\sqrt3}{\sqrt4}\to\cos\theta=\pm\dfrac{\sqrt3}{2}\\\\\theta\in[0^o,\ 90^o],\ \text{therefore all functions have positive values or equal 0.}\\\\\cos\theta=\dfrac{\sqrt3}{2}](https://tex.z-dn.net/?f=%5Cleft%28%5Cdfrac%7B1%7D%7B2%7D%5Cright%29%5E2%2B%5Ccos%5E2%5Ctheta%3D1%5C%5C%5C%5C%5Cdfrac%7B1%7D%7B4%7D%2B%5Ccos%5E2%5Ctheta%3D1%5Cqquad%5Ctext%7Bsubtract%7D%5C%20%5Cdfrac%7B1%7D%7B4%7D%5C%20%5Ctext%7Bfrom%20both%20sides%7D%5C%5C%5C%5C%5Ccos%5E2%5Ctheta%3D%5Cdfrac%7B4%7D%7B4%7D-%5Cdfrac%7B1%7D%7B4%7D%5C%5C%5C%5C%5Ccos%5E2%5Ctheta%3D%5Cdfrac%7B3%7D%7B4%7D%5Cto%5Ccos%5Ctheta%3D%5Cpm%5Csqrt%7B%5Cdfrac%7B3%7D%7B4%7D%7D%5Cto%5Ccos%5Ctheta%3D%5Cpm%5Cdfrac%7B%5Csqrt3%7D%7B%5Csqrt4%7D%5Cto%5Ccos%5Ctheta%3D%5Cpm%5Cdfrac%7B%5Csqrt3%7D%7B2%7D%5C%5C%5C%5C%5Ctheta%5Cin%5B0%5Eo%2C%5C%2090%5Eo%5D%2C%5C%20%5Ctext%7Btherefore%20all%20functions%20have%20positive%20values%20or%20equal%200.%7D%5C%5C%5C%5C%5Ccos%5Ctheta%3D%5Cdfrac%7B%5Csqrt3%7D%7B2%7D)
Answer:
Step-by-step explanation:
Use substitution, y=3x+18 so -12x+4(3x+18)=-24. From here you can try to solve for x
-12x+12x+72=-24
The xs cancel, which leaves you with 72=-24
Since 72 cannot equal -24 there is no answer to this system of equations
15(25-5a)=4-a
375-75a=4-a
Add 75a to both sides
375= 4+74a
Subtract 4 from both sides
371=74a
Divide both by 74
5.01=a
Pretty sure the first one is C
