ABCD is a square with side a. In centres A,B,C and D four circles are drawn such that each circle touches externally two of the three remaining circles. Let δ be the area interior of the square and exterior of the circles. What is the value of δ?
δ = Shaded area δ = Area of a square - (4 x ¼ area of a circle) = a² - (area of a circle) = a² - πr² but r = ᵃ⁄₂ = a² - π(ᵃ⁄₂)² = a² - ((πa²) ÷ 4) = (4a² - πa²) ÷ 4 = a² [(4 - π) ÷ 4]
ABCD is a square with side a. In centres A,B,C and D four circles are drawn such that each circle touches externally two of the three remaining circles. Let δ be the area interior of the square and exterior of the circles. What is the value of δ?
δ = Shaded area δ = Area of a square - (4 x ¼ area of a circle) = a² - (area of a circle) = a² - πr² but r = ᵃ⁄₂ = a² - π(ᵃ⁄₂)² = a² - ((πa²) ÷ 4) = (4a² - πa²) ÷ 4 = a² [(4 - π) ÷ 4]
Secure video calling is in high demand. As an alternative to Zoom, many people are using end-to-end encrypted apps such as WhatsApp, FaceTime or Signal to speak to friends and family face-to-face since coronavirus lockdowns started to take place across the world. There’s another option—secure communications app Telegram just added video calling to its feature set, available on both iOS and Android. The new feature is also super secure—like Signal and WhatsApp and unlike Zoom (yet), video calls will be end-to-end encrypted.
Telegram Auto-Delete Messages in Any Chat
Some messages aren’t supposed to last forever. There are some Telegram groups and conversations where it’s best if messages are automatically deleted in a day or a week. Here’s how to auto-delete messages in any Telegram chat. You can enable the auto-delete feature on a per-chat basis. It works for both one-on-one conversations and group chats. Previously, you needed to use the Secret Chat feature to automatically delete messages after a set time. At the time of writing, you can choose to automatically delete messages after a day or a week. Telegram starts the timer once they are sent, not after they are read. This won’t affect the messages that were sent before enabling the feature.
