Даны векторы a→(2;1){"color":"#202335","fontFamily":"stix","language":"ru"}и b→(2;-4){"color":"#202335","fontFamily":"stix","language":"ru"}. Найдите скалярное произведение векторов a→+b→{"color":"#202335","fontFamily":"stix","language":"ru"} и 7a→-b→{"color":"#202335","fontFamily":"stix","language":"ru"}.
