ข้อสอบคณิต 9 สามัญ - ปี 2560

ข้อ 15

กำหนดรูปสี่เหลี่ยม ABCD ดังรูป

โดยมีด้าน BC comma AC และ AD ยาวเท่ากับ 5 comma 7 และ 8 หน่วยตามลำดับ           

มี straight B straight A with overbrace on top straight D equals 90 degree และ straight C straight B with overbrace on top straight A equals 120 degree

พื้นที่ของรูปสามเหลี่ยม ACD เท่ากับข้อใดต่อไปนี้

รีวิว - เสียงตอบรับจากผู้เรียน

เฉลยข้อสอบ

       
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space space space space space space space space space space space space space space space space space space space space space space italic a equals 5 space comma space italic b equals 7

ส ู ตร space cos italic i italic n italic e space space italic b squared space equals space italic a squared plus italic c squared minus sign 2 italic a italic c space cos space B
space space space space space space space space space space space space space space space space space space space space space space 7 squared space equals space 5 squared plus italic c squared minus sign 2 open parentheses 5 close parentheses open parentheses italic c close parentheses space cos space 120 degree space space space space space space space space space
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begin mathsize 12px style space space space space space space space space space space space space space space space space space space space space space space sin space 30 degree space equals space fraction numerator open vertical bar C F close vertical bar over denominator open vertical bar B C close vertical bar end fraction
space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space 1 half space equals space fraction numerator open vertical bar C F close vertical bar over denominator 5 end fraction
space space space space space space space space space space space space space space space space space space space space space space space space space open vertical bar C F close vertical bar space equals space 2.5

จะได ้ space space space space space space space space space space space space space space space space open vertical bar C E close vertical bar space equals space open vertical bar C F close vertical bar plus open vertical bar F E close vertical bar
space space space space space space space space space space space space space space space space space space space space space space space space space open vertical bar C E close vertical bar space equals space open vertical bar C F close vertical bar plus open vertical bar A B close vertical bar
space space space space space space space space space space space space space space space space space space space space space space space space space open vertical bar C E close vertical bar space equals space 2.5 plus 3
space space space space space space space space space space space space space space space space space space space space space space space space space open vertical bar C E close vertical bar space equals space 5.5 end style

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