If a matrix is not invertible, can i say the matrix has many solutions? or is the matrix has many solutions as well as the possibility or no solutions? and so if a matrix is invertible, the system of equations will definitely only have one unique solution?

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?

At the beginning of the year, adrian's savings account balance was \$28$28. each week, he deposits another \$15$15 into that account, and he doesn't spend any of his savings. is his savings account balance proportional to the number of weeks since the start of the year?

Marking brainliest for original answer.. are these two triangles similar? how can you tell?