17. Equation cosa =tana is equivalent to cosa - sin a =0. Let s= sin a then we can cos a rewrite this equation into a quadratic equation of s as Without solving, we know sin a>0 since so we look for a positive root and find in the context s= (keep two decimal places after the decimal point). 18. For function f (x)=3cos (4x+(x/2)) , the period is and the phase-shift is