book Maths Puzzle Numerical Ability

how do you solve:
is triangle DEF, d=5 e=6 f=3 then COS E=????

i have the right set up i believe but i cant get the right answer helpp!!!

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book Other Question

half the perimeter of a building is 100 feet. the length of the building is 20 feet more than the width. find the dimensions of the building? I need your help on changing word phrases to mathematical sentences, I need your answers as soon as possible.