Answer:
See below.
Step-by-step explanation:
[tex]\cos(20)-\sin(20)=\sqrt{2}\sin(25)[/tex]
First, use the co-function identity:
[tex]\sin(90-x)=\cos(x)[/tex]
We can turn the second term into cosine:
[tex]\sin(20)=\sin(90-70)=\cos(70)[/tex]
Substitute:
[tex]\cos(20)-\cos(70)=\sqrt{2}\sin(25)[/tex]
Now, use the sum to product formulas. We will use the following:
[tex]\cos(x)-\cos(y)=-2\sin(\frac{x+y}{2})\sin(\frac{x-y}{2})[/tex]
Substitute:
[tex]\cos(20)-\cos(70)=-2\sin(\frac{20+70}{2})\sin(\frac{20-70}{2})\\\cos(20)-\cos(70) =-2\sin(45)\sin(-25)\\\cos(20)-\cos(70)=-2(\frac{\sqrt{2}}{2})\sin(-25)\\ \cos(20)-\cos(70)=-\sqrt{2}\sin(-25)[/tex]
Use the even-odd identity:
[tex]\sin(-x)=-\sin(x)[/tex]
Therefore:
[tex]\cos(20)-\cos(70)=-\sqrt{2}\sin(-25)\\\cos(20)-\cos(70)=-\sqrt{2}\cdot-\sin(25)\\\cos(20)-\cos(70)=\sqrt{2}\sin(25)[/tex]
Replace the second term with the original term:
[tex]\cos(20)-\sin(20)=\sqrt{2}\sin(25)[/tex]
Proof complete.
