When you multiply radical expressions you multiply the numbers under the radical symbols.<span><span>3–√</span>×<span>7–√</span>=<span>21<span>−−</span>√</span></span> When you add radical expression you CANNOT add the numbers under the radical symbols. <span><span>3–√</span>+<span>7–√</span>≠<span>10<span>−−</span><span>√</span></span></span>