If a+b+c=0, prove that a^2 - b^2 - c^2 = 2bc

Aretha wanted to gather data about the cost of local bowling leagues in her area. she plotted the data and determined that the average bowling league costs consist of a one-time registration fee and a monthly fee modeled by the equation y = 15x + 20. identify and interpret the y-intercept in this model. the y-intercept is 20. this is the cost per month. the y-intercept is 20. this is the cost of registration. the y-intercept is 15. this is the cost of registration. the y-intercept is 15. this is the cost per month.

If tan2a=cot(a+60), find the value of a, where 2a is an acute angle

Sum of the interior angles of a pentagon