In Exercises $15-22,$ graph the integrands and use known area rommus evaluate the integrals.

$15.{\displaystyle \underset{-2}{\overset{4}{}}}(\frac{x}{2}+3)dx$

$16.{\displaystyle \underset{\frac{1}{2}}{\overset{\frac{3}{2}}{}}}(-2x+4)dx$

$17.{\displaystyle \underset{-3}{\overset{3}{}}}\sqrt[\phantom{2}]{9-x^2}dx$

$18.{\displaystyle \underset{-4}{\overset{0}{}}}\sqrt[\phantom{2}]{16-x^2}dx$

$19.{\displaystyle \underset{-2}{\overset{1}{}}}|x|dx$

$20.{\displaystyle \underset{-1}{\overset{1}{}}}(1-|x|)dx$

$21.{\displaystyle \underset{\phantom{}}{\overset{1}{}}}(2-|x|)dx$

$22.{\displaystyle \underset{\phantom{}}{\overset{1}{}}}(1+\sqrt[\phantom{2}]{1-x^2})dx$