a,b为不等向量,且3a+2b与5a-4b垂直,4a-5b与6a+5b垂直,那么a与b的夹角是多少?

(3a+2b)(5a-4b)=15a^2-2ab-8b^2=0
(4a-5b)(6a+5b)=24a^2-10ab-25b^2=0
75a^2-10ab-40b^2=0
51a^2=15b^2
15^2-2√(15*51)cosθ-8*51=0
cosθ=(61√85)/170