Make f the subject of the formula d= 3(1-f) / f - 4

Given: △abc, ab=15, bd=9 ad ⊥ bc , m∠c=30°. find: the perimeter of the triangle.

Why did the ice arena get so hot after the big game (this is math related google it to find the paper

[10 points, algebra 2-simplifying complex fractions]according to my answer key the restrictions are -1 and 4. but why not positive 1?