Cookies are on sale! Today each cookie costs \( \$ 0.75 \) less than the normal price. Right now if you buy 7 of them it will only cost you \( \$ 2.80 \) ! Write an equation to determine the normal price of each cookie \( (c) \).

The sum of the first three terms of an arithmetic sequence is 15 and their product is 80. Determine the first threc terms.

Make \( d \) the subject of the relation in \[ W=M \sqrt{\frac{\left(D^{2}+d^{2}\right)}{2 d^{2}}} \]

The ratio between the sum of the fifth, sixth and seventh terms of a geometric series ind the sum of the first three terms of the same series is \( 81: 16 \). Determine the sum he first ten terms, if the first term is 1024 .

275. Використовуючи властивості віднімання, спрости вираз: \( \begin{array}{ll}\text { 1) }(37+a)-12 ; & \text { 2) }(b+415)-300 ; \\ \text { 3) } 42-(x+13) ; & \text { 4) } 517-(412+y)\end{array} \)

\( 0.07=e^{\frac{-\pi x}{\sqrt {1-x^{2}}}} \)