7x+4y=15, y=-5x+20 solve each system by using substitution

Find the value of the cosine for angle a

If, while training for a marathon, you ran 60 miles in 2/3 months, how many miles did you run each month? (assume you ran the same amount each month) write your answer as a whole number, proper fraction, or mixed number in simplest form. you ran __ miles each month.

Show your workcan a right triangle have this sides? *how to prove it without using the pythagorean theorema*